If you want to clean out prefs then best is to remove prefs.js and redo your settings.

If you have more files in your current profile folder that have leftovers from a long past then maybe this is a good time to start with a fresh profile and clean out all remnants from old Firefox versions.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles

You can transfer files from a previously used profile to the new profile, but make sure not to carry over problems.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

You can copy specific files with Firefox closed to the current profile folder to transfer personal data.

• bookmarks and history: places.sqlite
• favicons: favicons.sqlite
• bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
• cookies.sqlite for the Cookies
• formhistory.sqlite for saved autocomplete Form Data
• logins.json (passwords) and key4.db (58+) or key3.db (57 and older) (decryption key) for Passwords saved in the Password Manager
  if you only have key3.db then make sure to remove an existing key4.db
• cert9.db (58+) or cert8.db (57 and older) for (intermediate) certificates stored in the Certificate Manager
  if you only have cert8.db then make sure to remove an existing cert9.db
• persdict.dat for words you added to the spelling checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
• sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)

Thanks. This is all stuff I was hoping to avoid facing. It is a shame there is no less laborious way to go about this.

If you edit prefs.js then you need to be very cautious not to corrupt the file like you notice and if there are really a lot of old preferences then chances to corrupt the file get higher and you can still leave prefs that are connected to a pref you removed. It is a lot of work to check what a specific pref is about to determine if it is safe to remove.

The current version of about:config can have either a reset button (left arching arrow) or a delete button (trash can) at the right end of each preference row.

If you filter on, say, extensions, or printer, then the ones which are not native Firefox preference but were generated based on an extension or discovery of a printer, should all have trash cans.

See also your other thread about chase.com

• /questions/1298238 Strange Results on https://www.chase.com/

Thanks! I did not notice the trash can icons in about:config. I can clean up some stuff that way.

Cheers

I have not created and migrated to an entirely new profile (at least not yet) as that seems too much of a struggle at this point. However, I did use the new trash can icon in about:config to cleanly delete a number of ancient entries (which I could multi-select and then hit the trash can once) and that alone was gratifying. In doing this I was also able to ID a number of files and folders in my profile that were tied to old addons/FF versions and removed them as well. Even bookmarks.html. So I feel better having done at least some house cleaning. Thanks!