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 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.