The most interseting feature of firefox is to add extra funnctionality and features through extensions. By extensions you can extends the capabilities of browser which makes firefox a leading browser.So I'm posting here about my favourite firefox extension :
Tabbrowser Preference: It allows you to open link in new tabs. Also add functionality to middle click to open links in new tab.But newer versions of firefox already have this feature.
FoxyTunes: It allows you to control any media player without leaving browser. It has many features like keyboard short-cuts, skinnable, volume control, Track information.
All-in-one-mouse-gestures: Allows you to play with browser using mouse-gestures (can be user defined) . It increases speed of browsing by customization , tab and history scrolling and many more navigation aids.
Googlebar:It gives you the advanced search features of google search like search images, search particular site, search via special google site( like linux search , dictionary search, scholar search). It also add features for Google blog.
Download Manager Tweak : The Download Manager Tweak extension replaces the default Firefox download manager with a customized version. The customized version allows you to open download manager either in a seperate window, the sidebar, or a new tab.
Web developers (for firefox as developer tool) :The Web Developer extension is a toolbar (among other tools) that allows you to visually see how a website is created. Using this extension, it is easy to view various information about the website, such as the size of images. It also allows you to outline elements of the site, such as tables and other block elements. If you develop web pages for a living or as a hobby, or simply want to learn more about HTML, XHTML and CSS definitely install this extension.
Updated list extended to include this : My Firefox extensions (2)
Firefox About Pages