Ruh-roh. I'm using FF5 and the Apple site's collapsing top menu is not working for me either.
Haven't had any other problems though.
Apple's website is not a benchmark for browsers at all, because Apple makes sure that their website only works well with Safari. It's the same strategy that Microsoft used to follow with their products Outlook Web Access and Sharepoint which only enabled all of their features on Internet Explorer.
That being said, I don't know what problems people hear seem to have with Firefox. Firefox has been my main web browser since Apple screwed up Safari - Safari has been a total mess since version 4, in my opinion. I've been using Firefox since version 3.x on Windows, Linux and Mac OS X on a daily basis, including all the betas of version 4 and now 5, and this browser is a good work horse. Its HTML 5 support lags behind Chrome, but since HTML 5 still is not an official standard, it doesn't really matter. Furthermore, most Intranet products that we use at work (e.g. WhatsUp Gold) support only IE and Firefox, all other browsers might work but there are no guarantees.
Firefox 4 has been working very well for me since its very first beta version and it's a good browser - even on Mac OS X. Heck, Firefox 5 even works great on Android! (And unlike the iPhone toy, Firefox 5 on Android is the real thing, they fully ported the browser to Android.)
Instead of complaining, people should be grateful that they get a free alternative to Apple's own browser. After all, OS X is a niche platform and it's not even one that's nice to develop for, Jobs made sure of that.