CoLT - CoLT (Copy Link Text) makes it easy to copy either a hyperlink's text or both the link and the link's text from the context-menu.
Works with: Firefox: 1.5 - 3.0pre

Dictionary Search - Highlight any word in your browser and look it up in an online dictionary of your choice.
Works with: Firefox: 1.5 - 3.0b3

Foxmarks - If you use Firefox on more than one computer, you'll want Foxmarks. Install Foxmarks on each computer, and it will work silently in the background to keep your bookmarks synchronized.
Works with: Firefox: 1.5 - 2.*

Googlepedia - Shows you a relevant Wikipedia article alongside your Google search results.
Works with: Firefox: 1.5 - 2.*

IE Tab - View any page in Internet Explorer as a Tab in Firefox with just one click. (well it says one but it's two really)
Works with: Firefox: 1.5 - 3.0pre

Screengrab - Screengrab saves entire web pages as images. It will save what you can see in the window, the entire page, just a selection or a particular frame... basically it saves web pages as images.
Works with: Firefox: 2.0 - 3.0a3

Tiny URL Creator - If you share URL's via email or a Blog then this takes a long URL as input, and gives you a short URL to use in it's place.
Works with: Firefox: 1.5 - 2.*

Video DownloadHelper - The easy way to download Web videos from hundreds of YouTube-like sites. Also works for audio and picture galleries.
Works with: Firefox: 1.5 - 3.0b5

Web Developer Toolbar - The Web Developer extension adds a menu and a toolbar to the browser with various invaluable web developer tools.
Works with: Firefox: 1.0 - 3.0pre