Download all the links, movies and audio clips of a page at the maximum speed with a single click, using the most popular, lightweight and reliable external download managers.
The Web Developer extension adds various web developer tools to the browser.
Xmarks is the #1 bookmarking and open tab add-on. Keep your bookmarks, and open tabs backed up and synchronized across computers and browsers. Mobile phones with Premium access. Utilize LastPass to sync passwords!
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
UnPlug is an extension which lets you save video and audio which is embedded on a webpage - it's a video download tool.
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)
Add useful extra features to Gmail, like hierarchical labels, an unread message icon on your browser tab, file attachment icons, and more.
Downloads all or selected embedded objects on a webpage. Can be used for downloading movies, mp3s, flash, quicktime, or other embedded files (although this does NOT work on video embedded in flash [ youtube, video.google.com, etc....
Saves a webpage completely, providing a better alternative to the flawed built-in save functionality of Firefox.