Mozilla have put a tremendous amount of effort into the "ugly on every platform" cross platform tools over the years, from XUL to whatever the current one is. Developing a native UI for each platform would have been less effort and given much better results than continuing with this tried and failed approach.
Their latest attempt at theme support is just laughable and was very obviously not QA'd. In text areas I get the dark background theme from my desktop and the dark text foreground from the website css and it's made firefox unusable for me.
This is on top of things like their recent breakages to extensions so things that used to work (vertical tabs) still don't. And this only broke so they could go multi-threaded and then quickly had to add throttles so random sites couldn't hog every core.
But it seems mozilla are more concerned with re-writing things in rust, tying users into various web services and god knows what other side projects while they continue to avoid improving their core product.
I'd love to see firefox regain market share, we need an open source browser that's not controlled by a spyware company, but on their current trajectory firefox will dissapear into history and mozilla themselves will be the primary cause.