It's usually down to the remaining 10% to pick up the rest of the pieces.

It's extremely frustrating to hear this attitude as someone who was doing front-end dev in the IE6 era.

That's about the right ratio to expect, since less than 10% of browser sessions appear to be Firefox on average.

The major difference between this era and the IE6 era is that Microsoft wasn't willing to play ball with standards. With few exceptions, Mozilla and Google are implementing to the same standards and specifications; problem is, Chrome's a better implementation, for speed and stability, in the average case and the corner case.

They have both more resources to throw at the problem and more access to data to proactively discover issues.

It is, unfortunately, a situation where the playing field is "fair" (in the sense that there's a uniform standard) and one player is just trouncing the alternative.

This is not the interpretation I make of the current landscape. Google is SETTING standards that directly benefit themselves, just like MS did; Chrome uses it's market share to push in directions that benefit Google before they benefit end users.

Corner case maybe, but average case? No way. This is a historic perspective, maybe, but it's no longer valid in 2020.

Chrome isn't consistently the best of all browsers tested, but it beats FF more often than it doesn't.

1) site owner receives complaint

2) if offered, site owner looks up the user agent

3) site owner finds that the user agent matches one that is sub-10% of their traffic and samples the complaint signal accordingly

High-volume sites don't actually optimize for 100% of potential users (because the marginal value for one more user is low). If it's a low-volume site that happens to attract FF users for some reason, complaining is likelier to succeed.

Usually when I come across a broken website, it has more to do with my ad blocker and privacy extensions than the fact I'm using Firefox.