Firefox not Mac enough?

Firefox uses it's own parts instead of the ones that are in OS X already. Spellchecking is one of those things. If I'm correct this is due to the fact that Firefox is written in Carbon and not in Cocoa. I think Mozilla is working on those features, one of the next versions of Firefox (in the near future) should be able to use the spellchecking and services from OS X. They optimised a lot in Firefox 3.5 so it integrates much better in OS X than previous versions but they're not quite there yet although they really are working on getting there.

Firefox just works as it does on Windows and Linux, the only thing is that it can not use some features OS X offers such as the centralised keychain and spellchecker. This means that password you save in Firefox are only saved in Firefox' own system which in turn means that you can not use those passwords in Safari (or any other program that is able to use the keychain). Same thing goes for spellchecking. For you as a Linux user this won't change a bit but for OS X users who are used to these features this changes things (they have to do some more work than they're used to). No major flaws, just things that make life a bit easier.