This doesn't match my experience. I use Firefox as my "daily driver" browser on my personal machine and the number of browser compatibility issues I run into over a given period of time approximates 0. It's vanishingly rare for me to encounter something that's Firefox specific.

Curious to hear which sites you experience issues on.

I use a lot of SaaS admin panels, of widely varying quality. In every case I can think of, the same bugs are present in Firefox, Chrome, and Safari.