Take a full cold system backup before continuing and only if the above fails.
I’m all out of bright ideas then, except to uninstall firefox with the --no-save option (clearing all configuration files)
pamac remove --no-save firefox*
delete any remaining files too:
find / -iname \*firefox\* -print
Change -print to -exec rm {} \; if you want to actually delete them.
Reinstall firefox:
pamac install firefox
