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.

