<frameset> causes "TypeError: MutationObserver.observe: Argument 1 is not an object"

[mkeller]

FormHistoryControl log an error to the console when the user opens a page using the <frameset> tag.

To Reproduce

1. Go to https://www.tutorialspoint.com/html/html_frames.htm
2. See in browser console:

TypeError: MutationObserver.observe: Argument 1 is not an object. 3 [collectFormData.js:963:25](moz-extension://baedb3c4-9d4c-4c37-9592-7709714d6a1c/content/collectFormData.js)
    addBodyObserver moz-extension://baedb3c4-9d4c-4c37-9592-7709714d6a1c/content/collectFormData.js:963
    addAllHandlers moz-extension://baedb3c4-9d4c-4c37-9592-7709714d6a1c/content/collectFormData.js:1046
    <anonymous> moz-extension://baedb3c4-9d4c-4c37-9592-7709714d6a1c/content/collectFormData.js:1050
    inject resource://gre/modules/ExtensionContent.sys.mjs:581
    AsyncFunctionNext self-hosted:852

Expected behavior
No error. Support for <frameset>

Desktop

• Firefox, Windows 10

Fix
in collectFormData.js:964, replace

        aDocument.querySelector("body"),

by

        aDocument.body,

[stephanmahieu]

Fixed in v2.5.7.1, released 18 October.