Lets you set up keyboard shortcuts to launch sites, and, even better, bookmarklets.

"CustomizeGoogle is a Firefox extension that enhances Google search results by adding extra information (like links to Yahoo, Ask.com, MSN etc) and removing unwanted information (like ads and spam)."

Full-featured weblog editor in the form of a Firefox add-on.

Interesting Firefox add-on that lets you highlight text on a web page and do stuff with it - search, translate, convert currency, &c.

"YouTube Comment Snob is a Firefox extension that filters out undesirable comments from YouTube comment threads."

View PDFs in Firefox. Ridiculous that this isn't a standard feature.

Really slick.

A great new extension for mucking about with microformats - default settings are very obtrusive, though, so you need to switch everything off in prefs.

Rather slick-looking note-taking and research extension for Firefox.

"Click the toolbar button to go to the site that first referred you to the current site." Great for tab-crazy types.

Insanely handy Firefox extension that lets you trick the latest beta into working with your existing extensions. Most of the 20-odd I use appear to work fine with 2.0b2.