It's not clear what the reason for a backup browser is, despite looking at several replies.
I use only firefox, but on 2 different userids.
I have a "scratch" userid (I call it ff, for firefox) where I do all my normal browsing.  This is run from within firejail, and Ctrl-Shift-Delete is applied regularly.  JS is disabled using "Disable Javascript" extension, which has the nice feature that you can enable JS *by tab* -- very convenient when a site will get JS from arbitrary places and you still want it to work.
On the same userid, using different firejail settings, I run firefox -- effectively from a temporary and completely different $HOME -- for sites where I have to login, but I don't consider them critical.  Slashdot, reddit, ... you get the idea.
I have another, medium security, userid, from which I run firejailed firefox, which is used *only* for sites where I have to authenticate myself *and* the site is important.  Anything to do with money for instance.  Or gmail (on the rare occasions when I have to do something that cannot be done from Thunderbird), etc.  Note this is a completely different Linux userid, for additional separation.
No passwords are saved anywhere; I use a small shell script that gets stuff out of Jason Donenfeld's "pass" program (i.e., GPG protected password file).