I realised today that I actually had quite a few firefox extension installed, so while going through the list, I thought I’d put down my 10 favourite 
- Firebug: One of the best web development tools I have found. It allows you to debug and monitor CSS, HTML, and JavaScript live in any page. Tremendous help when programming in AJAX, to see the POST variables etc.
Download it here - The Web Developer Toolbar: It adds a menu and toolbar to firefox which enables you to do a lot of cool web dev stuff, like viewing a site with css, cookies, javascript disabled as well as resizing your browser to help you view what sites look like at different screen resolutions.
Download it here - Timestamp Converter: Any dev who has worked with timestamps will appreciate this. It’s already made my life a lot easier.
Download it here - IE Tab: As much as web developers (at least this one) dislike using IE, we have to accept the fact that the majority of web users out there still use IE as their browser of choice. While developing, I try and make sute that the site looks similar in both IE and firefix (more strict about this at work than at home :)).
Download it here - PDF Download: I have clicked on pdf file many times before without realising and have then had to wait for the file to download and load while my computer has a heart attack. This little addon was a god send to stop that. It warns the user when the user clicks on a pdf link and prompts to continue opening or saving.
Download it here - Foxmarks Bookmark Synchronizer: If you use firefox on multiple machines and want to synchronise your bookmarks, this is the addon for you.
Download it here - Video DownloadHelper: The easy way to download videos from you tube like sites.
Download it here - Adblock Plus: Block pesky ads
Download it here - Measure it: Allows you to draw a ruler across any webpage to check width and height of page elements in pixels.
Download it here - Abduction: Allows you to download an entire page or part of a page as an image. Download it here
