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.
Quicktext is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails.
Add symbol in header plane to switch between compact and expanded view. Options to switch between one or two lines compact view, which buttons should be displayed in header pane. Linkify RSS subject. Optionally darkens header if message has focus.
Search and remove messages which are the same as other messages in your mail folders (referred to as duplicates or dupes).
Extracts all attachments from selected messages and then can delete, detach or mark-read.
Powerful message searching through expressions. Type "from:fred to:tom" to see all messages from Fred to Tom in the current view. Support 'regular expressions' and 'click to search'.
The main goal allows users to open (and in some cases edit) page source with external applications.
Keyboard shortcuts to change folder, move/copy messages, with folder name auto-completion.
Quote Colors allows you to select text and background colors of different quote levels which are applied when reading or printing mail/news messages. Additionally, you can choose between graphical and traditional plain text quoting display mode.
Auto Zip Attachments allows you to easily compress (zip) attachments.