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.
Out of interest what part of the UI don't you like? You can drag and drop pretty much any button and component where ever you want and you can use the Firefox colours website to apply any colour scheme you want. This is all core browser features so no performance affects.
What is it that a theme is then adding?