The Web Developer extension adds various web developer tools to the browser.
FireFTP is a free, secure, cross-platform FTP/SFTP client for Mozilla Firefox which provides easy and intuitive access to FTP/SFTP servers.
Check out my new project, FireSSH!
Expand your email area by blocking and removing ads on the right hand side of the screen when using Gmail, Hotmail, Outlook.com and Yahoo Mail.
Image Block adds a toggle button that conditionally blocks/allows loading of images on webpages. Blocking images results in faster browsing, especially helpful on handheld devices or slower connections like GSM/GPRS/EDGE dial-up etc.
A clean, easy to use and highly extensible Internet Relay Chat (IRC) client.
Help make Firefox better. The Add-on Compatibility Reporter enables you to report if the Firefox add-ons that you're using are working properly.
A Statusbar with speed, percentage, time and loaded size (similar to Opera's one)
Hide ads in Gmail, and more cool stuff, simply with CSS.
Allows you to hide almost anything via context menu 'Remove Object'.