Toggle Proxy Requires Restart
Toggle Proxy adds a status bar/toolbar icon to toggle between two proxy settings which can be set in the preferences or toggle via a shortcut key (alt + x) if you don't use the add-on bar.
ToggleReplied Requires Restart
Enables toggling of Replied/Forwarded states of messages.
ToneQuilla Requires Restart
Adds a new action to mail filters to allow playing a particular sound when the filter matches. Also includes several sample sound files.
Toodledo Sync Requires Restart
ToodledoSync is an extension that I did for my need of a GTD system using Thunderbird and Toodledo.
IT IS NOT MAINTENED ANYMORE as I do not have any time. I'll give the (ugly) code to anyone willing to maintain it.
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.
Moves Edit > Preferences to Tools > Options…
traduku Requires Restart
Translate text from English and other languages into Esperanto.
Traduku tekston de la angla kaj aliaj lingvoj al Esperanto.
Trashcan Requires Restart
Have you ever accidentally deleted an event and didn't notice? Undo is no longer available? Then this extension is for you! All deleted items are put into a special Trashcan calendar for your safety.
Tv program Requires Restart
It shows popular stations TV program in listings, alerts and bubble infos. It's only for Czech and Slovak TV stations.
Localization: sk, cs, hu, en
Ubuntu Unity unread count and quicklist Requires Restart
Notify Unity of new mails in Thunderbird. Also, right click Thunderbird icon and get new quicklist items: Compose new mail and Contacts.
Based in awn-notif by Thesa aka Florian Birée
UMTools Requires Restart
Add new options to TB's Unsent Messages behavior.. ...
Unifinder Views Requires Restart
Add more views to the unifinder (event list) in Sunbird and Lightning.
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.
Unread Count Requires Restart
Maintains a file in the profile root directory with counts for unread messages per account. Useful for display in generic status bar, e.g.: taffybar, xmobar, dzen2, etc
Update Notifier Requires Restart
Note from Mozilla: This add-on has been discontinued. Try Add-on Update Checker instead.
Notifies you when updates are available for your extensions and themes.
URL Link Requires Restart
URL Link is a small Firefox and Thunderbird extension that allows you to select a non-URL in a mail/news message or web-page, and open it in a browser window.
Block advertisement and access to adult sites
Use Bcc Instead Requires Restart
Who among us has not made the mistake of sending a 'broadcast' email to a large number of recipients without using BCC; thereby revealing everyone's email addresses to all? This simple add-on attempts to prevent this from occurring.
UxU - UnitTest.XUL Requires Restart
This is a testing tool for addon developers.