YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
Allows editing of web page content while in Print Preview mode, prior to printing or saving as HTML or PDF. Compacts the layout and removes unwanted content such as adverts, sidebars and blank pages. Any element can be formatted, hidden or deleted.
Debug Flash/SWF files on the web. Edit properties and inspect elements on the fly. View log output inside the extension. Run AS3 code against the active SWF. Access assets with the Explorer. View AMF calls and Shared Objects and much much more...
Displays tabs in tiled layouts in Firefox. Arrange tiles horizontally, vertically or in a grid. Resize tiles by dragging splitter bars. Tabs and links can be dragged to open in other tiles. Supports multiple tiled layouts. Works with tab groups.
3D in your browser! FoxTab brings innovative 3D functionality to your Firefox.
New! Top Sites for FoxTab (aka Speed Dial).
AutoPager automatically loads next pages when you reach the end of a page. It works on a ton of sites,Google,Yahoo..
It works well with most other add-ons adblock plus, WOT and most of the greasemonkey scripts.
You can also try AutoPagerLite.
FirePHP enables you to log to your Firebug Console using a simple PHP method call.
Not using Firebug 1.9+ on Firefox 8+? See http://firephp.org/
A Statusbar with speed, percentage, time and loaded size (similar to Opera's one)
FirePath is a Firebug extension that adds a development tool to edit, inspect and generate XPath 1.0 expressions, CSS 3 selectors and JQuery selectors (Sizzle selector engine).
(former Google Dictionary and Google Translate)
In-line dictionary for any word you point to or double-click; whole page translation of any web page in another language.
Status bar widgets and progress indicators for Firefox 4+
TinEye is a reverse image search engine. The plugin adds a context menu item that allows you to search for an image to find out where it came from, how it is being used, if modified versions of the image exist, or to find higher resolution versions.