From the Nostalgy extension:
Here is how to set up your Thunderbird environment. You should firsy uninstall Nostalgy and exit Thunderbird. Then, the first step is to locate the extensions/ directory of your Thunderbird's profile. Under Unix, it is usually located in a directory like $HOME/.thunderbird/12345f.default/extensions. In this directory, you should see one sub-directory for each installed extension. Just create a new file in this extensions/ directory with a single line that contains the absolute path to the directory where your working copy has been created. The name of this file must be email@example.com.