I think you will have to use a method other than Mozilla to sign in to connect. I can use google and one other which I forget. Once you are signed in, you may be able to change your connected email account.

An user script of general application is one which changes the background colour of some sites. Also, for facebook only, FBPurity. https://greasyfork.org/en/scripts/979-nobrighter https://www.fbpurity.com/

I use bookmarklets also to change background colour. Two are based on a script posted on reddit, two others from https://www.squarefree.com/bookmarklets/.

I'm not seeing any such posts on fedia.

Evidently, adding a tag to a bookmark enables that section. In my case it's already there.

See also https://blog.mozilla.org/en/mozilla/local-translation-add-on-project-bergamot/

There's an issue with the Simplify Copilot extension which is being updated to fix it.