When Using Firefox PR1, I noticed that you can easily create new tabs if you keep the Tab bar open all of the time (Preferences -> Advanced -> Uncheck 'Hide the tab bar when only one site is open').
Just double-click on a blank area in the tab bar, and a new tab is created. I haven't tried this in previous versions, but it is a nice feature.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041002121829370