14 DAY TRIAL // Mozilla has announced that users of Thunderbird 3.1 have until April 24 to migrate to a newer version of the email client, since the old one will no longer be supported. All development will be halted at said date, which means that no security or performance fixes will be provided.
Truth be told, Thunderbird 3.1.x is pretty ancient, as the stable version was released in June 2010. The alternative could be the latest release, Thunderbird 10, which is also available as a portable application.
One of the biggest problems when upgrading is with extension compatibility, as some of the add-ons working on Thunderbird 3.1 may not be available for later versions, like it happened to me with MinimizeToTray Plus.
In this case you have to look for alternatives, and there should be plenty of them. In my case I found a replacement in MinimizeToTray revived.
Download the latest version of Thunderbird from here; portable version here.