You can’t log in to Backstabbr on Firefox without disabling enhanced tracking protection.
The good news is you can turn it off for this site only, log in, then turn the protection back on and you’ll remain logged in.
To turn off the enhanced tracking protection, look to the left of the address bar, and click on the shield. You’ll then be given a switch you can toggle.
I’m recording this here for future me, and/or anyone else who stumbles across this problem.
Thanks to the pseudonymous user shinythebald for pointing out this solution to me.