I work in IT and had to abandon Firefox because of compatibility issues that came up on a regular basis. it appears companies are simply not using it as part of their QA anymore. Also, in general the GUI theming has issues for me with the font and distinguishing highlights with my crappy vision. I tried every theme out there and for some reason apparently people writing themes just don't care to make it so you can see what is highlighted and what is not. Even The default theme sucks in my opinion. There were a number of other nits that I just kept having issues with - getting prompted on eBay to verify my identity for no reason, repeatedly, which doesn't happen on chromium and stuff like that.
I wish Apple would adopt the Firefox rendering engine and take Safari cross platform. It would give Firefox a fighting chance at the overall market.
Well, 1st, your sources are are weak here. However, it is a fact Brave is also run by con artists and swindlers.
The issue many users have is compatibility. Firefox zealots ignore the fact IT folks must work with Chromium. I cannot get the tools I need to work reliably on Firefox (or LibreWolf, Mullvad, etc.).
So, within the Chromium limitation, , I work on 7 systems regularly, I must have bookmark replication, MacOS/Linux/Windows/Android support:
Ungoogled Chromium = rough, no bookmark replication Vivaldi = worse than Brave, because no full source code Opera = Chinese Iridium = Indian Brave = source code available, privacy focused Edge = lol Chrome = lol
Winner? Brave. I use it with Pi-hole DNS on my home network, forced to use it with work DNS on their networks. I do also use LibreWolf (aka Firefox) with the Mullvad extension. I use it along with Brave, and hopefully at some point I can switch. I've tried 3 times in recent years, but too many web interfaces have Firefox issues, since it's blatantly not being used to QA websites anymore.