You wouldn't think so just from reading the bland name, but Preferences Toolbar is the best
Mozilla plug-in ever. It is also quite possibly the best
web browser plug-in ever, but I'm not an expert. As with the other
best things in life this puppy is
free.
Imagine the following scenario: You are trying to use Mozilla to view the world wide web and suddenly you find yourself on a page full of text that blends in with the background color, tiny illegible fonts, a useless CPU-hogging Java applet, a whole shedload of massive animated GIFs, a pop-up window full of advertising and some JavaScript that is screwing with your "Back" button. Oh, did I mention that the only way you know this is because you're borrowing someone else's machine because yours doesn't have Internet Explorer and the site flatly denies entry to any browser that doesn't identify itself as such? Yeah.
Unfortunately there is also a lengthy document on this page full of important links and other HTML that you really, really want to read.
Now ordinarily to view this godawful monstrosity in Mozilla first you would look at the source of the page denying you entry due to your browser choice and dig out the URL of the actual site from within its messy JavaScript. Having gotten past this bit of idiocy you would click on Edit, Preferences... then through four or five different sub-menus within the Preferences window to locate and uncheck all the various boxes that allow the site to abuse your computer and your eyeballs. Then of course when you were done with the ugly site you would be wanting to go back and re-check them all so that sites not created by color blind e-tards will display correctly. Not much fun, right?
Wouldn't it be nice if all those check boxes were on a toolbar conveniently located right in front of your face at all times? Wouldn't it be great if you could also use this toolbar to spoof Mozilla's user-agent and quickly access sites too stupid to realize that Mozilla can quite easily render them? While we're fantasizing, wouldn't it be incredible if this tool bar was almost infinitely configurable with not only the ability to quickly remove and re-arrange the check boxes, buttons and drop-down menus but also the ability to create your own controls to accomplish anything you can think of with XUL and Mozilla?
You don't have to imagine any more, Preferences Toolbar does all of this and much more. If you use Mozilla you can (and should) check it out:
http://prefbar.mozdev.org/