I've been dealing with a very slow Firefox 3.0.1 lately and just figured out how to cure this.
Thought the rest of you might want to try it and see if it helps your experiences with FF3.
There are several bug reports concerning heavy drive thrashing and slow operation. These all boil down to the way FF3 is handling some data.
To help people identify Phishing sites, forgeries (phorgeries?), and general ickiness, FF3 contacts Google.com for EVERY http request. EVERY REQUEST
It then saves the results into 'urlclassifier3.sqlite' in your profile.
This file grew to more than 50MB at which point I read that you could 'vacuum' it using 'sqlite3' to drop the unused database entries. I did this, it shrank to 33MB, a savings of 17MB... but the thrashing continued.
If you will disable the Google checks, then exit FF3 and delete the 'urlclassifier3.sqlite' file from your profile, your FF3 experience will be much nicer, albeit at the expense of some assistance in Phish Site Phinding. (mmm, Ben & Jerry's Phish Food Ice Cream...)
Turn off these two options: (under preferences)
Security -> "Tell me if the site I'm visiting is a suspected Attack site"
Security -> "Tell me if the site I'm visiting is a suspected forgery"
Close the preferences window, exit Firefox, then delete the 'urlclassifier3.sqlite' file:
rm .mozilla/firefox/*/urlclassifier3.sqlite
The file WILL be recreated upon the next start of Firefox3, but it's only 32KB in size, and isn't expected to grow since the two "Tell me" options are off.
It's made a world of difference in the browsing experience for me.
Hopefully this helps others.