How to get your device logs

Through Xcode

Have your phone connected to Xcode.
Open the devices and simulator window under XCode menu Window -> Devices and Simulator.
Select your device in the side bar list.
Click on View Device Logs.
Sort by processes to find Firefox related logs.

Launch Firefox.
Open Settings.
Scroll down to Version and tap it several times.
Scroll down and hit "Debug: copy log files to app container". This triggers the logs to be saved into the Documents folder.
Connect your device via USB.
Open Xcode.
Window > Devices. Choose your device.
Find "Firefox" on the right side.
Click the three dots icon, and choose "Download Container…". Save it somewhere you can find later. This saves a .xcappdata file.
Locate the .xcappdata file you just saved and right-click on it to "Show Package Contents".
Navigate to AppData/Documents/Logs, you have all the devices logs there.

Note that logs are currently separated in three categories:

Sync to record what's happening with Sync, Accounts, Providers, Storage, and Profiles
Browser to record what's happening in the front-end and the browser
Keychain to record interactions with the keychain