Ooh, shortcut key! Worth trying just for that.

"A Firefox extension produced as a collaboration between The Tor Project and the Electronic Frontier Foundation. It encrypts your communications with a number of major websites."

"A Google Chrome extension that helps you identify and fix performance problems in your web applications. It visualizes metrics that are taken from low level instrumentation points inside of the browser and analyzes them as your application runs. Using Speed Tracer you are able to get a better picture of where time is being spent in your application."

"Allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site."

"feedly weaves your favorite content into a fun, magazine-like start page.
based on Google Reader and Twitter."

"Adds a tab to Firebug for measuring the load time of web pages. "

Google's answer to ySlow.

Still experimental "A colour contrast analyser that examines the foreground and background colours of text nodes, and provides a report to see if they are correct according to AERT and WCAG's luminosity colour contrast algorithm."

"A prototype that encrypts and securely synchronizes the Firefox experience across multiple browsers, so that your desktop, laptop, and mobile phone can all work together. It currently supports continuous synchronization of your bookmarks, browsing history, saved passwords and tabs."

"An experimental add-on to explore geolocation in Firefox 3 ahead of the implementation of geolocation in a future product release. Geode provides an early implementation of the W3C Geolocation specification so that developers can begin experimenting with enabling location-aware experiences using Firefox 3 today, and users can tell us what they think of the experience it provides."

"A Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML. By toggling the composition on and off, the developer can visually see how many pixels they are off in development."

Less useful than the dev.mobi list.

"Creates screenshots of web pages [...] provides a set of editing and annotation tools, which let users quickly modify captures and insert text and graphical annotations." PC only.

"With gDocsBar, you can drag and drop multiple files into the sidebar to upload documents. You can search, filter documents right from the sidebar."

"NoSquint is a Firefox extension that allows you to adjust the default text zoom level, which is very useful if you have a small display or run at a very high resolution. NoSquint also remembers the zoom level per site (though you can disable that).

A developer tool for interacting with web services and other web resources that lets you make HTTP requests, set the entity body, and content type. This allows you to interact with web services and inspect the results.

"Provides an edit button for any edit box on any page"

"One-click access to various services that provide information about the web site you are viewing."

"Open pages in Opera from Firefox and Mozilla context menus."

"An extension for Showing and Exporting Microformats."

"Lets you easily validate documents from within Firefox 1.0 and Firefox 1.5 using the CSE HTML Validator Engine."

"View and modify HTTP/HTTPS headers and post parameters, trace and time http response/requests, security test web applications by modifying POST parameters."

"Replaces the behavior of Ctrl+Tab. Instead of directly changing tab it works some thing like Alt+Tab on windows."

"Allows to choose if you want to view a PDF file inside the browser (as PDF or HTML), if you want to view it outside Firefox with your default or custom PDF reader, or if you want to download it!"

"Renders a text version of a web page similar to screen reader output. Helps developers find accessibility issues fast."

"Allows you to check the page you are browsing in the waybackmachine archive."

"Allows you to submit a form to a new tab or window."

"Draw out a ruler to get the pixel width and height of any elements on a webpage."

"Adds HTML validation inside Firefox and Mozilla."

"Adds a menu and a toolbar with various web developer tools."

"Allows you to quickly move up a level in a site by pressing Alt + Up arrow."

"Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies."

"Adds context menu items for opening links and images in a new background tab (or foreground tab if your tabs normally open in the background), in a new background or foreground window, and in the current tab."

"Skype™ Voice Calls (as MP3), Instant Messages, Email, Appointments, Contacts all organized and under control in Outlook"

"How Mozilla Firefox can be used in conjunction with the Web Developer Toolbar to carry out a preliminary accessibility review"

"Simple UK streetmap search from context menu"

"As you glide the mouse over the page, you will see a red rectangle framing each element under the cursor. You will also see a little yellow caption showing the HTML element type and its class or id if they exist."

"Slogger is an extension for Mozilla Firefox which can create a customizable browser history, automatically store all pages you visit locally, log pages via various search providers, and more."