It's the only reason you don't see a lot more broken websites on Firefox, which would drive its market share even lower.

Lots and lots of Web development projects target Chrome, then make sure it works on Safari. In that second step, they accidentally fix a lot of bugs that'd show up on Firefox, too.

Not a lot of them are bothering to check Firefox directly these days. Hell, a decade ago it wasn't making the test list in tons of cases.