One of the nicest things about using the Firefox browser, apart from it's sheer elegance in design, is the ability to add new plug-ins or extensions in a hassle-free manner. While plug-ins are also available for Internet Explorer, the variety of extensions for Firefox, IMHO, far outstrips those for IE.
Some useful extensions:
1. Super DragAndGo: One of my favorites, and recently updated for Firefox 1.0, this extension allows you to drag any link on a webpage to clearspace and open the link as a new tab. Once you get used to this mode of working, and not having to deal with tabbing between multiple new windows, you won't be comfortable without it.. Even better, if you select a phrase/word and drag/drop it anywhere, it will submit the phrase to your installed search engines a open the results in a new tab. Dragging an image and throwing it anywhere results in the image being downloaded.
2. ieview: This extension adds a "View page in Internet Explorer" to the context menu. This is useful in viewing certain pages that do not display just right in Firefox. Alternatively, Firefox View lets you open a page in Firefox from Internet Explorer.
3. Blogging Extensions: A variety of extensions for convenient blogging of web pages you visit are available, including 'BlogThis' for Blogger.com
4. Down Them All: This extension allows you to download all/some of the links on a page in a single-click.
5. FoxyTunes: This allows you to control just about every popular music player like WMA, iTunes, WinAmp from within the browser.
6. Adblock: Allows you to filter/block web advertisements and other irritants
7. Foxylicious: This extension integrates your del.icio.us bookmarks into your browser bookmarks.
The online community is vibrant, constantly contributing new, cooler extensions. This means the browser constantly evolves, something larger products find difficult, with their emphasis on release cycles.