I use Chrome 90% of the time because Firefox is slow and has many bugs on video sites like 9gag. The screen goes black, the video loses vertical sync, etc. The same happens with Edge.
In my experience, the problem with Firefox's popularity is technical. I'll use Firefox more often if it improves. Before Firefox 3.6 (probably that version), Firefox was my most used browser, but after that version, Firefox started getting slower and more buggy. I switched to Chrome because IE was unusable on some sites.
I've never used Firefox much on Android, but when I did, it was slower than Chrome.
It's likely that if Firefox fixes the issues, they'll gain traction again, but right now, I don't see that happening. Mozilla's goals are different.