Best Thunderbird Extensions and Tweaks

Here's a few extensions and tips to help you optimise and customise the Mozilla Thunderbird email client.

Extensions

Tweaks

Colour Coded Message List
Using this sample userChrome.css file (instructions) you can customise your message list to show new messages highlighted in blue, replied messages with a subtle green and particularly useful, deleted IMAP messages are greyed out.
Thunderbird message list showing coloured rows.
Minimum Font Size
If you find that messages sometimes arrive in fonts too small to comfortably read, you can easily set a minimum font size:
Options > Fonts > Minimum Font Size (ensure you set it for all character encodings that you receive.)
Calendar
This can be added to Thunderbird with either the calendar extension or Sunbird standalone client. Find information and downloads at mozilla.org/projects/calendar. Alternatively, "Lightning" is the working project name for an extension to tightly integrate calendar functionality (scheduling, tasks, etc.) into Thunderbird.
Automatic Calendar Startup
You may find it convenient to have the calendar open automatically with Thunderbird. You can easily do this by adding a command line parameter. For example, create a shortcut that opens Thunderbird, and change its command from thunderbird.exe to thunderbird.exe -mail -calendar. (Alternatively, you can use just the -calendar parameter to start the calendar without starting Thunderbird.)
Compose/Reply/Forward as HTML
Like any responsible internet user, you have your default email format set to plain text. However for those rare occasions that HTML becomes necessary, you can easily toggle to it by Shift-clicking the toolbar buttons. So Shift-Compose will compose an HTML message, etc. (At time of writing, this inexplicably does not work for forward.)
Forward Several Messages as One
If you've set your default for forwarding to "Inline", and you want to forward a conversation or other series of emails, just ctrl-select the various messages and go Message > Forward As > Attachment.
Gmail-like labels in Thunderbird
See Chris Ilias' Blog: Gmail labels in Thunderbird.
Palm/PocketPC/iPod Synching
Only iPod synching of the address book is available as far as I know. Otherwise these features do not appear to be currently available for Thunderbird, however they are underway at palmsync.mozdev.org and pocketpcsync.mozdev.org.
What Sound Effect Should I Use For New Mail Notification?
Well of course, none, because new email disrupts your productivity don't cha know? But if I cannot talk you out of it, perhaps you'd like to use my current new mail notification. (You may need to turn your speakers up a bit.)

Themes

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This list was compiled by db with contributions from others in this blog post and this forum thread. Feedback or suggestions via either of these channels is welcome.