Firefox is not a good citizen on any OS other than Windows.
In Linux, the Firefox button becomes an awkward-looking upside-down tab in the tab bar, because you're apparently not supposed to intrude into the sacrosanct title bar. (What about Chrome?) It also takes quite a bit of tweaking on the distributor's part to make Firefox look and behave like the rest of the OS (e.g. icon theme and font anti-aliasing), though this has gotten better in recent releases.
But most Apple apps like Safari and iTunes aren't good citizens on Windows, either.
Anyway, I think we should have a bit of leniency on web browsers. Part of the reason for the perceived incompatibility is that a web browser is not an ordinary app. A web browser is an environment in which most people spend most of their time. It's almost like an OS in itself, with its own UI paradigms such as tabs and add-on bars. Therefore browser designers put in a lot of effort trying to polish their UI exactly the way they want, and it's not easy to reconcile that with the native UI of several different OSes. It'll take time, especially since all the OSes are in the process of revolutionizing their own UIs.