I still use Firefox, but on macOS it's a second class citizen. There was a HN post months ago about Safari being the new "IE6" (sorry, can't find that one). But when it comes to UI, on macOS, Firefox is the new IE6. And people see the UI, and web devs test against Safari even with its flaws, so they experience is still good.

In general, my UI gripes:

* Bookmarks dialog from the 90s, various usability issues

* History dialog, same

* Lack of Tab Groups, still, and no plugin has given me the experience as Safari

* Profile switcher UI is also terrible

* Still no vertical tabs

* Still nothing like Safari "tab overview"

* Miscellaneous UX fails like not being able to hover over a Tab preview and close it without switching to it (like the 'x' on List All Tabs) or close a tab by hovering over a persistent x without switching to it

* Create bookmark dialog is limited so a small size and can't fit more than a few location folder entries

* macOS : after all these years, still can't full-screen like a normal macOS app (all app bar toolbars / chrome shift when mouse)

I can't remember an interesting UI feature in at least several years.