Thunderbird add-ons that I've either used for years, or tested and highly suggest.
20 973 997 gebruikers
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
1 104 440 gebruikers
Organize your schedule and life's important events in a calendar that's fully integrated with your Thunderbird email. Manage multiple calendars, create your daily to do list, invite friends to events, and subscribe to public calendars.
738 343 gebruikers
Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
311 970 gebruikers
Adds some tools to import and export folders and messages
182 566 gebruikers
This extension allows you to export your saved passwords and disabled login hosts using XML or CSV files that can be imported later.
435 980 gebruikers
Personas are free, easy-to-install "skins" for Firefox. Personas Plus extends that built-in functionality to give you even more control and easier access to new, popular, and your own favorite Personas.
90 991 gebruikers
Theme Font & Size Changer lets you change the global font size and font family used for the user interface in Firefox. Design your browser with your favorite font.
150 280 gebruikers
OpenPGP message encryption and authentication for Thunderbird and SeaMonkey.
92 294 gebruikers
Adds the ability to create and edit entries in the password manager.
143 527 gebruikers
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.
181 432 gebruikers
Allows URI texts written in webpages to be loaded by double clicks.
44 358 gebruikers
This 15KB restartless add-on will add a "File" -> "Restart" menu item and a ctrl/cmd + alt + r hotkey to restart Firefox or Seamonkey.
141 745 gebruikers
DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.