It's tricky, because now that Firefox is a (small) minority browser I often assume any breakage is Firefox-specific. But at least 80% of the time, when I retry something in Chrome, it's still broken. I usually don't, mentally file it as "broken in Firefox", and decide I don't need to use that site for now anyway. So my estimation of Firefox-specific breakage is probably much higher than the reality. A lot of the web is just plain broken, for all browsers, much of the time.

The one major category of real problems I run into are related to captchas.