Well, from the looks of it, it's gonna be Firefox 3 week at Greasy PC, and I couldn't be happier. I'm still getting to know this new browser, but from the looks of it we're gonna be as good of friends as the last one, if not better.
First thing's first: are all of your extensions enabled for use in FF3? When you install FF3 over FF2 it'll notify you of which extensions are incompatible with FF3, and which aren't. If possible, it'll search for updated versions of the software (as was the case with uber-extension FireFTP). In my case I'd say half of my extensions were deemed incompatible and were immediately disabled.
First, check the add-on's official page whether or not there are actually FF3 compatible versions. Popular Firefox extension Tab Mix Plus hasn't officially released a FF3 compatible version, but you can find the latest builds here, where I got the latest, compatible version. Greasemonkey has the same issue, and you can find their latest builds here.
When you've done all you can to get every extension you can compatibility-ized (that's a word), and you still have a bunch of extensions that aren't cutting it, it's time to make Firefox 3 stop being so picky. The following tweak suppresses Firefox's warnings and disablings (that's a word, I swear!) of not-fully-compatible extensions (thanks to Lifehacker).
First, type about:config in your location bar. Firefox'll politely remind you that this might cause problems with your browser, so you better be real careful, or you'll have some 'splaining to do to that penguin.
Next, right click anywhere, and create a new Boolean string. Call it extensions.checkUpdateSecurity and set it to False.
When you restart your browser, you should be able to re-enable the rest of your extensions. I suggest you do this one at a time in case one of those extensions actually does cause problems (I didn't have any problems, myself).
Regardless, here's hoping those extensions get themselves up to speed soon!