Did you have add-ons in Firefox? I almost never have sites break—certainly not anywhere near 15% of the time. And at least 95% of the time I've had issues in the past, it has turned out to be an add-on causing problems, rather than the browser itself. (Ad blockers would be the first thing to check in the case of random breakages.)

I don't think 15% is accurate, I only know of 1 site that breaks under Firefox and that is operated at minimum budget by a chinese electronics shop. I've literally never found any other site that broke on Firefox.

I suspect more detail regarding your setup would prove to be useful for context as neither one of your bullets point towards an issue with Firefox directly.