Firefox's CSS just feels very dirty, like its very tricky to figure out how to best change this or that element because of how many moving parts there are between state changes (like :hover) and how variables interoperate. And in some places to make a single change you have to change multiple variables, like if you want to change the height of the URL bar row.
I've spent many hours on the CSS, and each time I revisit it, I find that I could've wrote this or that better, and I looked for variables that the UI uses, and wonder why I didn't find them the first time. I've been writing CSS on and off for 8 years now, I wonder if I'm just bad it since I've never done web dev full time, or it's actually a lot of work to write good CSS.