One notable recent improvement I will mention is better support for dark mode in the default theme (although I think this fix precedes Firefox 89, it's definitely available in Firefox 88). I previously had to hack the CSS to prevent Firefox pages from searing out my eyes with white at night.

These days I'm more satisfied with the default CSS on e.g. the Firefox Home page, although some pages like "view source" still do not respect dark mode unless I edit them.