1. Speed.
2. Lack of interest in macOS. I still remember when I got the first Mac with a Retina Display, shortly after release, and most third-party apps were pixelated and looked like crap. Among browsers, the timeline looked like this:
June 11, 2012: Hardware announced and released; Safari HiDPI support
July 31, 2012: Chrome HiDPI support (50 days after release)
January 8, 2013: Firefox HiDPI support (211 days after release)
Safari obviously had the advantage of prior knowledge, but that doesn't excuse Firefox taking so much longer than Chrome.
Maybe they've improved things since then, but... considering that they still don't support the pinch-to-zoom trackpad gesture, I'm not optimistic.
3. Security. Firefox has been the only browser to not feature sandboxing for quite a few years. Now that Electrolysis has been revived, this should be fixed eventually.