While the Firefox extensions I've listed above are the ones I most recommend and use, there are a few others which warrant some mention.
PDF Download
The Portable Document Format (PDF) is a popular way to distribute documents. PDFs retain the formatting of the original document, and print quite nicely. But one of the problems with PDFs on the Web is that they an take a long time to open in your browser window, and it can take just as long to get back to the originating page by clicking the Back button.
That's where the PDF Download extension comes in. When you click on the link for a PDF, this extension asks you whether you want to download the file, open it in a new Firefox tab, or have the extension convert the PDF to HTML, and open the resulting file in a new tab.
Cute Menus
In most software, Firefox included, the menus are pretty dull. You click, say, File and all you see is the text of the menu items. Cute Menus, though, brightens up Firefox's menus by adding Windows XP-style icons beside each menu item.
While Cute Menus doesn't do anything that make your Web browsing better, it does make the Firefox interface just that much more attractive. This extension is subtle, but it quickly grows on you.
Quick Note
If you use the Web for research, or if you find that you're frequently jotting down information from Web sites, then Quick Note is for you. It's a little note pad that you can pop up beside Firefox to take down information. And using Quick Notes beats having to flip between Firefox and a word processor or an editor like Notepad. On top of that, you can save your notes as a text file for future use.