For an 'untrustworthy' browser (my FF beef is simply aesthetics) it has the most secure addons and extensions of any linux browser I've used. Mostly crash-fee is nice, too.
Dial in the right combo and you surf pretty safely--compared to the herd-- if you believe the footprint investigator sites (whoer, panopticon etc).
I know nothing of Mozilla's politics or privacy, nor has my reading led me there, so I'm not making a judgement on that based on lack of research. In advanced settings I elected not to share any data with them. If they circumvent that, oh well.
The hardening required to become truly anonymous seems too much a chore if all your interests are is solus, football, rock and roll, your old lady, and keeping your car running. And football.