MultiZilla is a Mozilla enhancement group who originally came up with the tabbed browsing concept now seen in Mozilla. It was mentioned on this site way back in 2001, but you may wish to check it out again.
What it is:
Multizilla is a toolbar and preferences pane that allows you quick access to things like changing user-agent strings and turning off referrer headers. It also has a lot of customization options for tabbed windows, such as "Close other tabs," "Show LED status lights on tabs," and "Show tab bar at bottom of window," among others.
Installation:
Just click on the installation tab of the above site and then on the install link for the Mac version. You need to have a recent version of Mozilla (I'm using 1.2b and it worked fine). Warning: Don't Install the "Google" thing.
Un-installation:
Follow these directions on the MultiZilla FAQ page
Additional Info:
More on how to set up user agent spoofing.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021127065227447