Дополнения

FlashGot Mass Downloader от 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.

User Agent Switcher от chrispederick

The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.

Session Manager от Michael Kraft

Функциональный менеджер сессий. Сохраняет и восстанавливает состояние всех окон.

British English Dictionary от Mark Tyndall

British English Dictionary R1.19 for Firefox, Thunderbird and SeaMonkey by David Bartlett.

To enable, right-click in a multi-line text box, select "Languages", then "English / United Kingdom".

Toolbar Buttons от Button Guy

Adds toolbar buttons to the customize toolbar window in several programs including Firefox, Thunderbird and SeaMonkey. Some of the buttons make commonly preformed actions quicker, others add new functionality.

Примечание подборщика totaltoolbar, not toolbar Buttons :\ http://totaltoolbar.mozdev.org/

Scriptish от Erik Vold, Nils Maier, другие

The greatest user script engine on the Internet (a fork of Greasemonkey).

NewsFox от Andrey Gromyko, R Pruitt

NewsFox является эффективным 3-панельным клиентом для работы с Atom/RSS (наподобие email-клиента)

QuickDrag от Stevie Smith

Extends the function of the drag-and-drop mouse gesture so that it can be used to load URLs, do a web search of selected text on a page, or save an image on a page. The successor to Super DragAndGo.

CuteButtons от ChoGGi

Adds icons to buttons and/or menus.

FxIF от Christian Eyrich, Ted Mielczarek

View EXIF data in image properties

BBCodeXtra от flod

(Sorry, no plans to port this add-on to Webextensions)
BBCodeXtra is an extension, compatible with Mozilla FireFox (up to 57) and Seamonkey, which adds to the context menu new commands to insert BBCode/Html/XHtml codes in an easy and fast way...