Ed. Note: Perhaps a word of warning? Discuss.
Firefox 3 appears to have issues with user accounts not located on the main drive. At the very least it cannot deal with a network user account being located on a remote OS X Server.
When I updated to Firefox 3, I immediately noticed that Bookmarks were not visible under bookmarks menu. The Search engine field had a generic icon and when I selected 'Manage Search Engines', the dialog box was frozen and I couldn't get out of it without quitting Firefox. When I tried to enter a URL into the URL field and press 'enter', nothing happens. However, when double-click on a URL in an e-mail message, that appears to work.
I initially ran Firefox 3 on a client Mac OS X 10.5.3. My home directory (and thus my Firefox profile) is on an XServe running OS X 10.5.3 Server.
When I switched to a local admin account (i.e., Firefox profile on the local hard drive), it seems to work fine. However, when I switch back to my network home account (on our XServe), it still displays the problems described above. I tried other user accounts on our XServe with the same problems.
Downgrading to Firefox2 solved the problems with no apparent permanent damage to the Firefox user profile.
As is, Firefox3 is completely unusable for my set up. I'll stay with Firefox2 until this is resolved.