Threadvis is an application that gives a visual indication about the thread the message belongs to.
It is a very fast way to look at the conversation an email is part of.
This add-on seems to work on Thunderbird 3.0.5 on Linux Mint 8.
The only thing was that it didn't register as installed.
The add-ons menu was keeping the 'restart to install threadvis' notification.
Looking at the profile i noticed that the directory in the profile of thunderbird was owned by root.
So i opened up an Xterm (via Alt-F2 and entering xterm)
did:
cd ~/.thunderbird
cd
.default
cd extensions
When i did a:
sudo chown -R \{A23E4120-431F-4753-AE53-5D028C42CFDC\}
And started Thunderbird again, it started up and worked.
Hope this is a pointer for you if your threadvis isn't installing.