diff --git a/cli/CHANGELOG.md b/cli/CHANGELOG.md
index 87ef9285d4d5..06821745825a 100644
--- a/cli/CHANGELOG.md
+++ b/cli/CHANGELOG.md
@@ -6,6 +6,7 @@ _Released 11/19/2024 (PENDING)_
 **Features:**
 
 - Updated the protocol to be able to flex logic based on project config. Addresses [#30560](https://github.com/cypress-io/cypress/issues/30560).
+- Added new [`defaultBrowser`](https://docs.cypress.io/app/references/configuration#Browser) configuration option to specify the default browser to launch. This option only affects the first browser launch; changing this option after the browser is already launched will have no effect. Addresses [#6646](https://github.com/cypress-io/cypress/issues/6646).
 
 **Bugfixes:**
 
diff --git a/cli/types/cypress.d.ts b/cli/types/cypress.d.ts
index f0f0c914b1c3..ab1dba589fe2 100644
--- a/cli/types/cypress.d.ts
+++ b/cli/types/cypress.d.ts
@@ -3216,6 +3216,11 @@ declare namespace Cypress {
     setupNodeEvents: (on: PluginEvents, config: PluginConfigOptions) => Promise<PluginConfigOptions | void> | PluginConfigOptions | void
 
     indexHtmlFile: string
+
+    /**
+     * The default browser to launch if the "--browser" command line option is not provided.
+     */
+    defaultBrowser: string
   }
 
   interface EndToEndConfigOptions extends Omit<CoreConfigOptions, 'indexHtmlFile'> {
diff --git a/packages/config/__snapshots__/index.spec.ts.js b/packages/config/__snapshots__/index.spec.ts.js
index c366c3230ee0..d119255c38cc 100644
--- a/packages/config/__snapshots__/index.spec.ts.js
+++ b/packages/config/__snapshots__/index.spec.ts.js
@@ -26,6 +26,7 @@ exports['config/src/index .getDefaultValues returns list of public config keys 1
     'specPattern': '**/*.cy.{js,jsx,ts,tsx}',
     'indexHtmlFile': 'cypress/support/component-index.html',
   },
+  'defaultBrowser': null,
   'defaultCommandTimeout': 4000,
   'downloadsFolder': 'cypress/downloads',
   'e2e': {
@@ -117,6 +118,7 @@ exports['config/src/index .getDefaultValues returns list of public config keys f
     'specPattern': '**/*.cy.{js,jsx,ts,tsx}',
     'indexHtmlFile': 'cypress/support/component-index.html',
   },
+  'defaultBrowser': null,
   'defaultCommandTimeout': 4000,
   'downloadsFolder': 'cypress/downloads',
   'e2e': {
@@ -206,6 +208,7 @@ exports['config/src/index .getPublicConfigKeys returns list of public config key
   'chromeWebSecurity',
   'clientCertificates',
   'component',
+  'defaultBrowser',
   'defaultCommandTimeout',
   'downloadsFolder',
   'e2e',
diff --git a/packages/config/src/options.ts b/packages/config/src/options.ts
index 87bfa61496d0..fd0afbc5e5eb 100644
--- a/packages/config/src/options.ts
+++ b/packages/config/src/options.ts
@@ -169,6 +169,10 @@ const driverConfigOptions: Array<DriverConfigOption> = [
       indexHtmlFile: 'cypress/support/component-index.html',
     },
     validation: isValidConfig,
+  }, {
+    name: 'defaultBrowser',
+    defaultValue: null,
+    validation: validate.isString,
   }, {
     name: 'defaultCommandTimeout',
     defaultValue: 4000,
diff --git a/packages/config/test/project/utils.spec.ts b/packages/config/test/project/utils.spec.ts
index 0a3ee14c76b8..a958047b54e8 100644
--- a/packages/config/test/project/utils.spec.ts
+++ b/packages/config/test/project/utils.spec.ts
@@ -1054,6 +1054,7 @@ describe('config/src/project/utils', () => {
             browsers: { value: [], from: 'default' },
             chromeWebSecurity: { value: true, from: 'default' },
             clientCertificates: { value: [], from: 'default' },
+            defaultBrowser: { value: null, from: 'default' },
             defaultCommandTimeout: { value: 4000, from: 'default' },
             downloadsFolder: { value: 'cypress/downloads', from: 'default' },
             env: {},
@@ -1152,6 +1153,7 @@ describe('config/src/project/utils', () => {
             browsers: { value: [], from: 'default' },
             chromeWebSecurity: { value: true, from: 'default' },
             clientCertificates: { value: [], from: 'default' },
+            defaultBrowser: { value: null, from: 'default' },
             defaultCommandTimeout: { value: 4000, from: 'default' },
             downloadsFolder: { value: 'cypress/downloads', from: 'default' },
             execTimeout: { value: 60000, from: 'default' },
diff --git a/packages/data-context/src/DataContext.ts b/packages/data-context/src/DataContext.ts
index 895dc4d12e4e..7f614bd67827 100644
--- a/packages/data-context/src/DataContext.ts
+++ b/packages/data-context/src/DataContext.ts
@@ -83,7 +83,7 @@ export interface GraphQLRequestInfo {
 export class DataContext {
   readonly graphqlRequestInfo?: GraphQLRequestInfo
   private _config: Omit<DataContextConfig, 'modeOptions'>
-  private _modeOptions: Readonly<Partial<AllModeOptions>>
+  private _modeOptions: Partial<AllModeOptions>
   private _coreData: CoreDataShape
   readonly lifecycleManager: ProjectLifecycleManager
 
@@ -122,7 +122,7 @@ export class DataContext {
     return new RemoteRequestDataSource()
   }
 
-  get modeOptions () {
+  get modeOptions (): Readonly<Partial<AllModeOptions>> {
     return this._modeOptions
   }
 
@@ -425,4 +425,8 @@ export class DataContext {
       this.#awaitingEmptyRequestCount.push(resolve)
     })
   }
+
+  updateModeOptionsBrowser (browser: string) {
+    this._modeOptions.browser = browser
+  }
 }
diff --git a/packages/data-context/src/actions/BrowserActions.ts b/packages/data-context/src/actions/BrowserActions.ts
index a78dd045cff9..35177f563a4f 100644
--- a/packages/data-context/src/actions/BrowserActions.ts
+++ b/packages/data-context/src/actions/BrowserActions.ts
@@ -40,6 +40,11 @@ export class BrowserActions {
     })
   }
 
+  setCliBrowser (browser: string) {
+    this.ctx.updateModeOptionsBrowser(browser)
+    this.ctx.coreData.cliBrowser = browser
+  }
+
   async focusActiveBrowserWindow () {
     await this.browserApi.focusActiveBrowserWindow()
   }
diff --git a/packages/data-context/src/data/ProjectLifecycleManager.ts b/packages/data-context/src/data/ProjectLifecycleManager.ts
index c46d10b5161d..16fc41f79a90 100644
--- a/packages/data-context/src/data/ProjectLifecycleManager.ts
+++ b/packages/data-context/src/data/ProjectLifecycleManager.ts
@@ -308,10 +308,25 @@ export class ProjectLifecycleManager {
   /**
    * Sets the initial `activeBrowser` depending on these criteria, in order of preference:
    *  1. The value of `--browser` passed via CLI.
-   *  2. The last browser selected in `open` mode (by name and channel) for this project.
-   *  3. The first browser found.
+   *  2. The value of `defaultBrowser` in `cypress.config`.
+   *  3. The last browser selected in `open` mode (by name and channel) for this project.
+   *  4. The first browser found.
    */
   async setInitialActiveBrowser () {
+    const configDefaultBrowser = this.loadedFullConfig?.defaultBrowser
+
+    // if we have a default browser from the config and a CLI browser wasn't passed and the active browser hasn't been set
+    // set the cliBrowser to the defaultBrowser from the config since we want the defaultBrowser to behave as if it was passed via CLI
+    if (configDefaultBrowser && !this.ctx.modeOptions.isBrowserGivenByCli && !this.ctx.coreData.activeBrowser) {
+      this.ctx.actions.browser.setCliBrowser(configDefaultBrowser)
+    }
+
+    // if we already have an activeBrowser, that means we are reloading the browser (e.g. after a config change in open mode)
+    // so we need to set the CLI browser to the activeBrowser to ensure the GUI shows the correct browser
+    if (this.ctx.coreData.activeBrowser && !process.env.CYPRESS_INTERNAL_E2E_TESTING_SELF) {
+      this.ctx.actions.browser.setCliBrowser(`${this.ctx.coreData.activeBrowser.name}:${this.ctx.coreData.activeBrowser.channel}`)
+    }
+
     if (this.ctx.coreData.cliBrowser) {
       await this.setActiveBrowserByNameOrPath(this.ctx.coreData.cliBrowser)
 
diff --git a/packages/data-context/test/unit/data/ProjectLifecycleManager.spec.ts b/packages/data-context/test/unit/data/ProjectLifecycleManager.spec.ts
index b526cc529194..07fcd4e5965c 100644
--- a/packages/data-context/test/unit/data/ProjectLifecycleManager.spec.ts
+++ b/packages/data-context/test/unit/data/ProjectLifecycleManager.spec.ts
@@ -2,12 +2,13 @@ import { expect } from 'chai'
 import type { DataContext } from '../../../src'
 import { createTestDataContext } from '../helper'
 import sinon from 'sinon'
-import { FullConfig } from '@packages/types'
+import { FoundBrowser, FullConfig } from '@packages/types'
 
 const browsers = [
   { name: 'electron', family: 'chromium', channel: 'stable', displayName: 'Electron' },
   { name: 'chrome', family: 'chromium', channel: 'stable', displayName: 'Chrome' },
   { name: 'chrome', family: 'chromium', channel: 'beta', displayName: 'Chrome Beta' },
+  { name: 'firefox', family: 'firefox', channel: 'stable', displayName: 'Firefox' },
 ]
 
 let ctx: DataContext
@@ -15,9 +16,6 @@ let ctx: DataContext
 function createDataContext (modeOptions?: Parameters<typeof createTestDataContext>[1]) {
   const context = createTestDataContext('open', modeOptions)
 
-  context.coreData.activeBrowser = undefined
-  context.coreData.cliBrowser = undefined
-
   context._apis.browserApi.getBrowsers = sinon.stub().resolves(browsers)
   context._apis.projectApi.insertProjectPreferencesToCache = sinon.stub()
   context.actions.project.launchProject = sinon.stub().resolves()
@@ -42,6 +40,9 @@ describe('ProjectLifecycleManager', () => {
 
   context('#setInitialActiveBrowser', () => {
     it('falls back to browsers[0] if preferences and cliBrowser do not exist', async () => {
+      ctx.coreData.activeBrowser = null
+      ctx.coreData.cliBrowser = null
+
       await ctx.lifecycleManager.setInitialActiveBrowser()
 
       expect(ctx.coreData.activeBrowser).to.include({ name: 'electron' })
@@ -51,6 +52,7 @@ describe('ProjectLifecycleManager', () => {
     it('uses cli --browser option if one is set', async () => {
       ctx._apis.browserApi.ensureAndGetByNameOrPath = sinon.stub().withArgs('electron').resolves(browsers[0])
 
+      ctx.coreData.activeBrowser = null
       ctx.coreData.cliBrowser = 'electron'
 
       await ctx.lifecycleManager.setInitialActiveBrowser()
@@ -68,6 +70,7 @@ describe('ProjectLifecycleManager', () => {
 
       ctx._apis.browserApi.ensureAndGetByNameOrPath = sinon.stub().withArgs('electron').resolves(browsers[0])
 
+      ctx.coreData.activeBrowser = null
       ctx.coreData.cliBrowser = 'electron'
 
       await ctx.lifecycleManager.setInitialActiveBrowser()
@@ -79,6 +82,8 @@ describe('ProjectLifecycleManager', () => {
 
     it('uses lastBrowser if available', async () => {
       ctx.project.getProjectPreferences = sinon.stub().resolves({ lastBrowser: { name: 'chrome', channel: 'beta' } })
+      ctx.coreData.activeBrowser = null
+      ctx.coreData.cliBrowser = null
 
       await ctx.lifecycleManager.setInitialActiveBrowser()
 
@@ -88,12 +93,84 @@ describe('ProjectLifecycleManager', () => {
 
     it('falls back to browsers[0] if lastBrowser does not exist', async () => {
       ctx.project.getProjectPreferences = sinon.stub().resolves({ lastBrowser: { name: 'chrome', channel: 'dev' } })
+      ctx.coreData.activeBrowser = null
+      ctx.coreData.cliBrowser = null
 
       await ctx.lifecycleManager.setInitialActiveBrowser()
 
       expect(ctx.coreData.activeBrowser).to.include({ name: 'electron' })
       expect(ctx.actions.project.launchProject).to.not.be.called
     })
+
+    it('uses config defaultBrowser option if --browser is not given', async () => {
+      ctx = createDataContext({
+        project: 'foo',
+        testingType: 'e2e',
+        isBrowserGivenByCli: false,
+      })
+
+      ctx._apis.browserApi.ensureAndGetByNameOrPath = sinon.stub().withArgs('chrome').resolves(browsers[1])
+      sinon.stub(ctx.lifecycleManager, 'loadedFullConfig').get(() => ({ defaultBrowser: 'chrome' }))
+
+      expect(ctx.modeOptions.browser).to.eq(undefined)
+      expect(ctx.coreData.cliBrowser).to.eq(null)
+      expect(ctx.coreData.activeBrowser).to.eq(null)
+
+      await ctx.lifecycleManager.setInitialActiveBrowser()
+
+      expect(ctx.modeOptions.browser).to.eq('chrome')
+      expect(ctx.coreData.cliBrowser).to.eq('chrome')
+      expect(ctx.coreData.activeBrowser).to.eq(browsers[1])
+    })
+
+    it('doesn\'t use config defaultBrowser option if --browser is given', async () => {
+      ctx = createDataContext({
+        project: 'foo',
+        testingType: 'e2e',
+        browser: 'firefox',
+        isBrowserGivenByCli: true,
+      })
+
+      sinon.stub(ctx.lifecycleManager, 'getFullInitialConfig').resolves(fullConfig)
+      ctx._apis.browserApi.ensureAndGetByNameOrPath = sinon.stub().withArgs('firefox').resolves(browsers[3])
+      sinon.stub(ctx.lifecycleManager, 'loadedFullConfig').get(() => ({ defaultBrowser: 'chrome' }))
+
+      expect(ctx.modeOptions.browser).to.eq('firefox')
+      expect(ctx.coreData.cliBrowser).to.eq('firefox')
+      expect(ctx.coreData.activeBrowser).to.eq(null)
+
+      await ctx.lifecycleManager.setInitialActiveBrowser()
+
+      expect(ctx.modeOptions.browser).to.eq('firefox')
+      expect(ctx.coreData.cliBrowser).to.eq('firefox')
+      expect(ctx.coreData.activeBrowser).to.eq(browsers[3])
+    })
+
+    it('ignores the defaultBrowser if there is an active browser and updates the CLI browser to the active browser', async () => {
+      ctx = createDataContext({
+        project: 'foo',
+        testingType: 'e2e',
+        isBrowserGivenByCli: false,
+      })
+
+      sinon.stub(ctx.lifecycleManager, 'getFullInitialConfig').resolves(fullConfig)
+      ctx._apis.browserApi.ensureAndGetByNameOrPath = sinon.stub().withArgs('chrome:beta').resolves(browsers[2])
+      // the default browser will be ignored since we have an active browser
+      sinon.stub(ctx.lifecycleManager, 'loadedFullConfig').get(() => ({ defaultBrowser: 'firefox' }))
+
+      // set the active browser to chrome:beta
+      ctx.actions.browser.setActiveBrowser(browsers[2] as FoundBrowser)
+
+      expect(ctx.modeOptions.browser).to.eq(undefined)
+      expect(ctx.coreData.cliBrowser).to.eq(null)
+      expect(ctx.coreData.activeBrowser).to.eq(browsers[2])
+
+      await ctx.lifecycleManager.setInitialActiveBrowser()
+
+      expect(ctx.modeOptions.browser).to.eq('chrome:beta')
+      expect(ctx.coreData.cliBrowser).to.eq('chrome:beta')
+      expect(ctx.coreData.activeBrowser).to.eq(browsers[2])
+    })
   })
 
   context('#eventProcessPid', () => {
diff --git a/packages/server/lib/modes/index.ts b/packages/server/lib/modes/index.ts
index c542b7d807b9..99b7085e8e5f 100644
--- a/packages/server/lib/modes/index.ts
+++ b/packages/server/lib/modes/index.ts
@@ -10,6 +10,8 @@ export = (mode, options) => {
     return require('./smoke_test').run(options)
   }
 
+  options.isBrowserGivenByCli = options.browser !== undefined
+
   if (mode === 'run') {
     _.defaults(options, {
       socketId: random.id(10),
diff --git a/packages/types/src/modeOptions.ts b/packages/types/src/modeOptions.ts
index f8ab17f81ec2..f12a99399f24 100644
--- a/packages/types/src/modeOptions.ts
+++ b/packages/types/src/modeOptions.ts
@@ -24,6 +24,7 @@ export interface RunModeOptions extends CommonModeOptions {
   parallel?: boolean | null
   ciBuildId?: string | null
   tag?: (string)[] | null
+  isBrowserGivenByCli: boolean
 }
 
 export type TestingType = 'e2e' | 'component'
diff --git a/system-tests/__snapshots__/results_spec.ts.js b/system-tests/__snapshots__/results_spec.ts.js
index bcc4f872c47e..a81d683240e7 100644
--- a/system-tests/__snapshots__/results_spec.ts.js
+++ b/system-tests/__snapshots__/results_spec.ts.js
@@ -16,6 +16,7 @@ exports['module api and after:run results'] = `
     "blockHosts": null,
     "chromeWebSecurity": true,
     "clientCertificates": [],
+    "defaultBrowser": null,
     "defaultCommandTimeout": 4000,
     "downloadsFolder": "/path/to/downloadsFolder",
     "env": {},
diff --git a/system-tests/projects/config-defaultBrowser/cypress.config.js b/system-tests/projects/config-defaultBrowser/cypress.config.js
new file mode 100644
index 000000000000..4276ccd434fc
--- /dev/null
+++ b/system-tests/projects/config-defaultBrowser/cypress.config.js
@@ -0,0 +1,6 @@
+module.exports = {
+  e2e: {
+    supportFile: false,
+  },
+  defaultBrowser: 'chrome',
+}
diff --git a/system-tests/projects/config-defaultBrowser/cypress/e2e/spec.cy.js b/system-tests/projects/config-defaultBrowser/cypress/e2e/spec.cy.js
new file mode 100644
index 000000000000..434d054dfaa1
--- /dev/null
+++ b/system-tests/projects/config-defaultBrowser/cypress/e2e/spec.cy.js
@@ -0,0 +1,3 @@
+it('works', () => {
+  expect(1).to.eq(1)
+})
diff --git a/system-tests/test/config_spec.js b/system-tests/test/config_spec.js
index b71c4a206a37..4668c329c8d4 100644
--- a/system-tests/test/config_spec.js
+++ b/system-tests/test/config_spec.js
@@ -243,4 +243,17 @@ describe('e2e config', () => {
       snapshot: true,
     })
   })
+
+  it('launches browser using config.defaultBrowser', async function () {
+    await Fixtures.scaffoldProject('config-defaultBrowser')
+
+    return systemTests.exec(this, {
+      project: 'config-defaultBrowser',
+      command: 'cypress',
+      args: ['run', '--dev', '--project', path.resolve(process.cwd(), './projects/config-defaultBrowser')],
+      onStdout: (stdout) => {
+        expect(stdout).to.include('Browser:        Chrome')
+      },
+    })
+  })
 })