I recently tried to move to Firefox (again). I try to to this every couple years. I run into the same three blockers every time. 1. The omnibar completion isn't as good. Chrome figures out where I go pretty regularly, and autocompletes extremely well based upon habits. 2. The dev tools aren't as good. I mostly work in the frontend, and simple things like how you write css overwrites with tab completion is just not as…
I switch to Firefox about once a year (seasonal surveillance paranoia?). I mostly run older machines and there's a noticeable performance difference between FF and Chrome-based browsers. As a compromise I run Brave (chrome-based, un-googled, and kind of privacy oriented?) but not really happy about that.
Fwiw I really want Firefox to succeed, I donate to Mozilla every month and I am learning Rust.
I think it's partly sequencing, eg. opening a new window in FF, FF waits until the window is built before showing it (takes nearly a second on my machine), while Chrome will open it instantly and then paint in the UI. The actual time til usable URL bar is the same on both, but on FF the experience is "I gave a command and nothing is happening... oh there it is".
There's just no indication that anything is happening until it's done, and that makes a big difference. It makes it feel slow: even if the total delay is exactly the same, the perceived delay is several orders of magnitude greater.
There's also some difference in the time it takes to load a page, but once they're loaded JS perf is on par.
I expect the difference would be less dramatic on a machine from this decade, but I haven't had the pleasure of finding out :)