I switched to Firefox recently and many sites don't quite work: for example the pull request popup menu on GitHub appears off screen so can't be clicked on; the "new post" panel in Discourse is obstructed by the keyboard; FastMail alert box buttons don't work, and many other such annoyances.
It can be used as a main browser but it does have problems. I wouldn't bother with it if it wasn't for the manifest v3 situation
The same workflow works fine in Chrome.
You seem to be talking about the mobile version?
This lead developers to switch from IE/Firefox where you had to install a plugin for developers, into using Google Chrome that had a native and more advanced tool for debugging.
*For me this is the key*.
If you provide a browser that greatly improve the developer's experience, they (the developers) will move to use it, making it the best compatible browser to use, which will then make the rest of the crowd to use it because the overall experience is better there.
I just hope the team at Firefox will realize this before Google ...
But.
I've been using both Firefox and Brave as my browsers, and Firefox is eating my memory way more than Brave (I have around 60+ tabs open at Brave, and just a few at Firefox). (I'm on Linux Fedora 36 with the latest versions available in the repository).
This is why I'm not using it much, but it saddens me.