I kinda think that Firefox Sucks So Much because their userbase just "accepts" unconventional aspects of Firefox without REAL thought behind it.
With RESPECT towards members here, we have that "blind acceptance" even HERE.
I realize that I'm in the minority, doesn't mean I can't be heard.

I cite Firefox's background DNS (merely as a RECENT discussion herein).
These "background" connections, regardless of how "trivial" or data-centric, FORCE software firewall rules to be OPENED WIDER.
Things like "loopback" rules. I will
not go into detail, research yourself! This has been a Firefox "negative" for over TWENTY YEARS.
If you use very strict parent-process-based firewall rules, Firefox has to be opened WIDER than browsers without these "trivial" DNS connections.
Call me paranoid, DON'T CARE!
To me, Firefox users are COMPLACENT and MAKE UP EXCUSES when "called out".
Forgive me for saying so, but they see themselves as "different" and "going against the grain" and MAKE EXCUSES to label it as "right".
There are some NON-COMPLACENT users - ie, things like this --
https://www.windowscentral.com/software-apps/browsing/firefox-users-slam-mozilla-over-controversial-data-privacy-update
But the reality is that MOST users of Firefox really DON'T CARE, they have BLIND TRUST in Firefox and create excuses when called out on it.
"But it's the lesser of two evils", or something like that.
I can't speak towards COMPILING Firefox, I have done it, but I also didn't trust my output.
But then again, I didn't really trust the input "base code" either.