Powerful and user-friendly selector utility for selecting Web page elements and doing various actions on them. It can be used for cleaning up a page prior to printing it (by removing and isolating elements), and for web development.
https://addons.mozilla.org/en-US/firefox/downloads/file/39726/aardvark-2.97-fx.xpi

The first and only download manager/accelerator built inside Firefox.
https://addons.mozilla.org/en-US/firefox/downloads/file/29952/downthemall!-1.0.3-fx+tb+sm.xpi

Open Firefox with the current page or a selected link displayed in Internet Explorer.
https://addons.mozilla.org/en-US/firefox/downloads/file/30623/-1.0-fx-win.xpi

Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
https://addons.mozilla.org/en-US/firefox/downloads/file/37452/firebug-1.2.1-fx.xpi

Along with transferring your files quickly and efficiently, FireFTP also includes more advanced features such as: directory comparison, syncing directories while navigating, SFTP, SSL encryption, search/filtering, integrity checks, remote editing, drag & drop, file hashing, and much more.
https://addons.mozilla.org/en-US/firefox/downloads/file/35671/fireftp-1.0.2-fx.xpi

The original (though unofficial) Google toolbar for Firefox, with an emphasis on easy access to many types of specialized searches.
https://addons.mozilla.org/en-US/firefox/downloads/file/34510/googlebar-0.9.15.14-fx.xpi

Allows embedding Internet Explorer in Firefox tabs.
https://addons.mozilla.org/en-US/firefox/downloads/file/36071/ie_tab-1.5.20080823-fx-win.xpi

Check the validity of links on any webpage.
https://addons.mozilla.org/en-US/firefox/downloads/file/31657/linkchecker-0.6.3-fx.xpi

Converts text links into genuine, clickable links.
https://addons.mozilla.org/en-US/firefox/downloads/file/32848/linkification-1.3.5-fx+mz+sm.xpi

Use PDF Download to do whatever you like with PDF files on the Web. Regain control of them and eliminate browser problems, view PDFs directly in Firefox as HTML, and use the all-new Web-to-PDF toolbar to save and share Web pages as high-quality PDF files.
https://addons.mozilla.org/en-US/firefox/downloads/file/34070/pdf_download-2.0.0.0-fx.xpi

The easy way to download and convert Web videos from hundreds of YouTube-like sites. This works also for audio and picture galleries.
https://addons.mozilla.org/en-US/firefox/downloads/file/40527/video_downloadhelper-3.4-fx.xpi

Allows you to to view page source with external applications but also much more.
https://addons.mozilla.org/en-US/firefox/downloads/file/40743/viewsourcewith-0.4-fx+mz+tb+sm.xpi

Adds a menu and a toolbar with various web developer tools.
https://addons.mozilla.org/en-US/firefox/downloads/file/28678/web_developer-1.1.6-fx.xpi