Okay, I've got more.

View Rendered Source Chart is an alternative to the traditional "view source" window, but it shows the actual rendered HTML (not any javascript that might have created it on the fly) and it prettifies it. Handy.

DownThemAll is a builtin download manager. Its purpose is supposedly to be able to download every link on a page, but I just use it because it can queue up downloads. I often find myself needing to download large patches from software vendors, but Firefox doesn't have a way to queue them up, and also prevents me from downloading more than a few at a time, assuming that the web site doesn't. (Yes, I know I can change that, but this is a better solution.) With this, I can have it download one, then the next, etc., without me having to keep an eye on the downloads.

Platypus allows you to interactively edit web pages so that they display to your liking. There's also another very similar extension, Aardvark (which, incidentally, has a demo on its home page, allowing you to see what it does without having to install the extension). But Playpus also allows you to save your modifications as a Greasemonkey script, which means that you can save those changes so that they will remain whenever you visit the site. Neat!