So you need to install Flatpak to manage AppImages ? 
The results I posted are from Librewolf and Mullvad. These come “pre-hardened” out of the box.
Mullvad is basically TOR but without routing over TOR.
I cannot talk to “vanilla” Firefox. I haven’t used it for some time now. I know there are several user.js files out there (Arkenfox for example) to tweak them in one go for more privacy and fingerprinting resistance.
@UncleSpellbinder Oups, sorry, I just had a look to Gearlever on github, and saw the flatpak package… My bad ! 
Yeah I tried to do that. I am currently running a few changes in the user.js file, but ever since I got those done, I am now facing Cloudflare hosted sites challenges. I simply cannot access them.
Been using it for a while too and yeah it’s pretty good. Only thing I’d watch is how often they push security updates. But seems fine so far.