I've personally experienced hangs as long as a minute on various computers. Slow DNS lookups always seem to be the culprit. Until someone fixes Firefox so it doesn't completely lock up, your best bet is to prevent slow DNS lookups.
In some cases, my VPN connection ended non-gracefully and left my DNS configuration in a bad state, i.e. OS X still tries to connect to the DNS servers on the private network even though I'm no longer connected, and Firefox hangs until the connection times out. An easy solution is to switch network locations thereby resetting your configuration to a good state.
If you still experience problems, you might try switching DNS servers or even installing a local DNS server. I haven't needed to try the latter just yet.
Update: I finally broke down and enabled the local DNS server. I followed the instructions from Mac OS X Hints. Many of the comments claim the hint won't work, but it worked great for me. I went from frequent 3 to 4 second hangs to no hangs at all according to Spin Control.