Firefox OS Simulator is a test environment for Firefox OS. Use it to test your apps in a Firefox OS-like environment that looks and feels like a mobile phone. After installing it, go to Web Developer > Firefox OS Simulator to access its features.
Cookies manager to view, edit and create new cookies. It also shows extra information about cookies, allows edit multiple cookies at once and backup/restore them.
Allows the download manager to also open in a tab or sidebar, and adds some optional display changes.
The greatest user script engine on the Internet (a fork of Greasemonkey).
Adds the ability to create and edit entries in the password manager.
Control what gets sent as the HTTP Referer on a per-site basis.
Make any pages' search functionality available in the Search Bar (or "search box")...
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon
For Firefox 23+, you need 0.958 (under review)
Save an image of a web page (including Flash content) to a file. Web page screenshots may be captured interactively or from the command line. Options allow you to control the output format (PNG or JPEG), image file name, scaling, and much more.