1.Web developer`s toolbar
ColorZilla offers You an advanced eyedropper, colorPicker, page Zoomer and other colorful goodies.
This plugin displays the number of found errors of the site you are viewing.
High performance for web sites.
FireFTP is simply an FTP client built into the Firefox browser. Important thing it is free and you can flip to it just like any tab in your browser. https://addons.mozilla.org/en-US/firefox/addon/684
You can check out all the HTML tags right on the page without using the view page source option.
Looks for broken links on page, links that are not working anymore - returning 404 code.
With Speed Dial, you can easily access your most used websites.
11.The Cooliris 3D Wall
Simply the fastest and most stunning way to browse photos and videos from the Web or your desktop. Effortlessly scroll an infinite '3D Wall' of your content from Facebook, Google Images, YouTube, Flickr, and hundreds more.
12.Video DownloadHelper 4.6.4
The easy way to download and convert Web videos from hundreds of YouTube-like sites. This works also for audio and picture galleries. https://addons.mozilla.org/en-US/firefox/addon/3006
SenSEO is an extension checking the most important on-page-SEO-criteria and calculating a grade of how good your site fulfills these criteria. https://addons.mozilla.org/en-US/firefox/addon/9403
Page Speed is an open-source Firefox/Firebug Add-on. Webmasters and web developers can use Page Speed to evaluate the performance of their web pages and to get suggestions on how to improve them.
Just a simple extension to highlight links in a page according to nofollow / dofollow status.