I donate to the Mozilla foundation, and I love Firefox a ton. But I can't seem to like the UI by installing a theme, and when I change it to look better the browser slows to a crawl. Does it really matter all that much if I use Chromium?
P.S. To the people from my last post regarding something similar, Firefox was too slow, I'm sorry, but I use Vivaldi instead of Brave because Brendon Eich can suck my dick.
Use whatever browser works best for you. It is kind of dumb to stick to something you do not like just because of ideology.
That said both Firefox and Chromium based browsers have pros and cons, there is no best one. My primary browser at home is Firefox, at work I use Edge, I like both. Some time ago tried to fully move from Firefox to Vivaldi, but went back due to couple things that I preferred on FF.
Your main problem is probably 6GB physical memory. At least on Windows 10+ I would not be comfortable with less than 16GB. Hope you at least have SSD.
I use Linux, so system memory usage shouldn't be an issue