Network effect I'd say. If tomorrow opera, brave, edge switched to Firefox, people won't notice but Firefox share would rise dramatically. Extensions is another issue. Today, any tom dick and harry who want to create a browser extension only build for chrome "why not". Firefox comes from complaining users who don't want to switch otherwise they don't bother. Its the same chicken or egg thing. Low market share of Fire…
Once of the problems Firefox has that it's actually quite hard to do this. You can't "just" take WebKit or Blink and make your own UI for it. But you can build a Blink-based browser in literally a minute or two with basic C++ or Python skills.
I don't know what technical issues are preventing this, but IMHO this has been a major hurdle, and a huge strategic mistake.