[jorticka]

It goes deeper.

People learn over time that complex web apps work badly in Firefox, because developers mostly test in Chrome.

So they don't even bother trying it in Firefox.

[nairboon]

Maybe. Or maybe Firefox users have sophisticated ads & tracker blocker, sometimes even UA spoofing. Then it really depends on your analytics method, if these users show up at all. You might have a lot of FF users, but analytics tells you otherwise.