...if it's not updating itself. This is probably because you didn't do a proper "install", but downloaded and set it up by hand.

1. Go to the Mozilla Thunderbird download site, find a suitable Linux download, save to ~/Downloads.
2. Explode the archive producing a new subdirectory Thunderbird.
3. Find your old subdirectory, rename it to Thunderbird.version.
4. Move the newly downloaded and exploded subdirectory alongside it.
5. Launch Thunderbird.

The important settings were already save under ~/.thunderbird, so all your mail should still be in place and your account settings still working. You will probably have to fix your proxy settings:

Edit -> Preferences -> Advanced -> Network & Disk Space -> Connection Settings -> Configure Proxies to Access the Internet -> Use system proxy settings (or something like that).

You have to click in a cell to get the control handles. Click the arrow pointing up to add a row above, or down for adding a row below.

Click the circle with the x in it to delete the cell or row—you'll have to experiment to understand what this will do.