Add-ons

DownThemAll! by Nils Maier, Federico Parodi, others

The first and only download manager/accelerator built inside Firefox!

FlashGot Mass Downloader by Giorgio Maone

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.

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UnMHT by arai_a

UnMHT allows you to view MHT (MHTML) web archive format files, and save complete web pages, including text and graphics, into a single MHT file in Firefox/SeaMonkey.

Mozilla Archive Format, with MHT and Faithful Save by Christopher Ottley, Paolo Amadini

— Open web pages saved with Internet Explorer or other browsers (MHT)
— Save many tabs, video and audio reliably, in a single ZIP file (MAFF)
— Return to the original site you saved a page from, and more!

ReminderFox by Tom Mutdosch, Daniel Lee, others

ReminderFox displays and manages lists of reminders and tasks. ReminderFox uses easy-to-use lists with warnings/alarms to remind you of all your important appointments.
Synchronize ReminderFox with other calendars as well.

Collector's Note This is an incredibly well made and complete reminder/calendar. I advise this strongly to anybody who tends to lose track of time browsing.

Dafizilla Table2Clipboard by dafizilla

Mozilla applications allow to select rows and columns from a table simply pressing Control key and picking rows/columns with left mouse button.
The selection can be copied to clipboard but the original table disposition is lost making ugly results wh...

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Quick Locale Switcher by ppclick

Allows you to quickly switch to a different language (User Interface, Spell Checker Dictionary and Website content) in your Mozilla application.

UnPlug by h2g2bob

UnPlug is an extension which lets you save video and audio which is embedded on a webpage - it's a video download tool.

Transliterator by Alex Benenson

As-you-type phonetic conversion of text from one alphabet to another, as well as transliteration/detransliteration of selected text anywhere within documents. Supports multiple languages and custom transliteration schemes.

Mozilla Labs - Jetpack Prototype by Mozilla Labs

Jetpack Prototype is an experiment in making it easy to build Firefox add-ons using common web technologies.

Collector's Note As Geasemonkey, also adds features. Check some out : http://userscripts.org/jetpacks or create it yourself

BBCode by Jed Brown, mrtech

Adds BBCode, HTML, XHTML & Symbols formatting to the context menu for forums.