QuickDrag is a Firefox extension that extends the function of the drag-and-drop mouse gesture so that it can be used to load URLs, do a web search of selected text on a page, or save an image on a page—all with a simple drag-and-drop.

A Mozilla Firefox browser extension that inserts preview images (thumbnails) of web sites into the Google and Yahoo search results pages. Thumbnails are created and provided by GooglePreview servers.

IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox. This is a great tool for web developers, since you can easily see how your webpage displayed in IE with just one click and then switch back to Firefox.

The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. It is designed for Firefox, Flock  and Seamonkey, and will run on any platform that these browsers support including Windows, Mac OS  X and Linux.