about:preferences#home
Homepage and new windows (Firefox Home (default) or you can change that.
I’m curious why you can’t… I can.
So open Dolphin and browse to 1. A few days old snapshot ~/.mozilla/firefox and the other side to your home .mozilla firefox.
Rename your current firefox.CURRENT and copy the snapshot one in… start up firefox again.
Start firefox with a new profile… firefox -P
Or look at a new tab about:profiles and make a new one there.
Generally this shouldn’t be necessary if you just log into your Firefox, make a profile in your name, then sign in and sync…