OK, everybody knows cool extensions for Mozilla Firefox, but last week I've been browsing to see whether I could make my Thunderbird email client even more nice to work with. And it came out good. So, lets see what I found:

• Attachment Extractor: save attachments from multiple messages at once
• Display Mail User Agent Extension: very interesting plugin, as it shows the email client the sender used as an icon in the message heading
• Display mailing list header: useful for people reading [lots of] mailing lists, as it displays mailing list specific actions [like Help, Unsubscribe, Post] in the message heading
• Message Notes: annotate your messages. Didn't try it yet, but it seems useful
• Quote Colors [latest version]: Thunderbird already has a way of showing inline replies in a better way then just a bunch of "> > >"'s, but this extension highlights the background of the text in that color too. Customizable
• QuoteCollapse: collapse inline quotes [from replies etc] by default, so reading becomes a lot easier. Makes a nice combination with Quote Colors
• Stacked View Extension: alternative [fourth] way of the main window layout. I prefer the Vertical layout a bit better, but it's kinda nice
• Mouse Gestures: what else do you need? ;)
• Mozilla Calendar [latest version, plugin seems out of date]: calendaring plugin for Thunderbird. Being replace by Sunbird

BTW, check this Firefox bug [the IDN buffer overflow security issue], which can be worked around quite simply. Please do so.