Firefox lacks in the default UX area, just from the top of my head: Print to PDF is better in Chrome, comes with a preview. Search marks matches in the scroll bar. Opening a recently closed tab opens a new tab in Chrome instead of replacing the existing one. There were probably 5-10 more annoyances in Firefox for me that I don't recall right now. Maybe it was my familiarity with Chrome but overall I still think Chrom…
Wait, isn't this something the OS print dialog is supposed to provide?
> Search marks matches in the scroll bar.
I'd like to see this implemented as well (just as another commenter mentioned).
> There were probably 5-10 more annoyances in Firefox for me that I don't recall right now. Maybe it was my familiarity with Chrome but overall I still think Chrome has better UX defaults.
Which is mostly because you're more familiar with Chrome overall (and it's also a matter of taste). As an example, I strongly dislike the downloads "bar" in chrome and the fact that bookmark/history/downloads pages are pages and not dialogs. Many people dislike dialogs tho, because they seem bloated and difficult to use to them.
One small UI gimmick I'd like to see implemented in major browsers (at least on macOS) is an option to use the fluent/vibrant blur tab bar background instead of the default white/gray/black. Firefox already kinda does that (at least when using the dark theme), but I still have to write my own userchrome.css to get the full effect.