People have problems with what they choose to program, not the quality of their code. I too have used FF since the beginning, but switched to Waterfox last year (it took me about two years to make that decision - I didn't make it lightly). I chose WF in large part because its profile remains compatible with FF so I can switch back if they calm the F down and start acting normal again for long enough to rebuild some trust.
https://en.wikipedia.org/wiki/Criticism_of_Mozilla_Corporati... - start at the end for most recent.
Also go to the website of any one of the FF forks and read their reasons for existing. For example: