Firefox implements useful features like spell checking and password storage. OS X users will be used to both since they're provided by the OS for use in all applications through a centralised dictionary and through the keychain. Firefox cleverly uses neither of these OS features, instead implementing its own solutions. So it uses its own dictionary, and its passwords don't show up in the Keychain Access application like those from all real OS X applications. To add insult to injury, the default download doesn't actually come with a dictionary — you need to download that separately. So Firefox stands alone in being the only OS X browser that doesn't spellcheck without additional downloads.

Previous versions of Firefox looked really horrible. The developers' solution seems to have been not to just use the native OS X widgets, but to try to emulate them. They've cleverly decided to try to duplicate the widget set circa maybe OS X v10.2, when probably 60% of current Mac owners didn't use a Mac, and even then the colours are noticeably off. Probably they're doing something stupid like ignoring the monitor's colour profile while they ignore all the native widgets.

Doing their own pixel painting also means that their browser seemingly doesn't work with any assistive technology, such as screen readers or the like. Try this: switch on the OS's built-in VoiceOver (it's in the Universal Access section of the System Preferences). Click on a text box in Safari or Safari's preferences and it'll read it out to you. Do the same in Firefox and it won't. That's because the Firefox developers are doing their own pixel pushing and ignoring the OS's way of doing things.

You might say that a lot of these complaints are unavoidable for cross-platform software. Well that's besides the point — they make for a very poorly implemented OS X application.