I use Librewolf on desktop and Mull on mobile. I have a few extensions on both, which could definitely contribute to issues. When I have issues (usually government sites or financial stuff, sometimes DRM-related stuff for media) it’s easier to just use a Chromium-based browser with no extensions than try to troubleshoot specifically what’s causing the issues. I keep Falkon (desktop) and Vanadium (mobile) installed for this purpose.
I get the feeling a lot of issues people are having in Firefox might be due to extensions or settings, which gets “fixed” by using another browser (which happens to be Chromium-based because most browsers are) and they blame the issue on Firefox itself.
While I’ll always be wary of corporations, Valve seems to be maintaining an overall good relationship with the FOSS community (for the time being).