Sorry I missed that.
You could try to install snap by adjusting/disabling apparmor.  But I doubt that will fix anything as the snap package for firefox will be the same as the firefox apt install (except with added dependencies).
Have you tried if firefox is actually working? (/path/to/firefox -headless)
Here is an interesting article: https://hacks.mozilla.org/2017/12/using-headless-mode-in-firefox/