Is there a reason why? Less funding? Web devs don’t make the pages Firefox friendly? Since the user base is smaller, they just don’t care?
Yeah, if that’s what OP means (though that’s unclear), I’m not sure why OP thinks it’s a bad thing. It’s a good thing.
Or maybe OP means Firefox crashes more or something. In which case I can only say that hasn’t been my experience.
My experience has been, however, that Firefox is quite usable on a Raspberry Pi 4 while Chromium is far too resource hungry to be usable on that platform.