I've never played around much with Firefox browser themes, but I found one that's actually useful: Littlefox. As I'm one to experiment with lots of buttons and toolbars, the clutter's usually unbearable. Littlefox shrinks all of that so that each button and toolbar takes up the least amount of vertical space while still being readable. If your eyes aren't completely shot, this will minimize scrolling and allow for easier screenshotting if you're one to do that for presentations.
If you have any other favorite Firefox themes or add-ons, be sure to share them in the comments.