The Whats New or Migration Assistant tabs keep appearing
When Thunderbird is updated from a prior major version (e.g. 2.0 to 3.0), a Migration Assistant tab is displayed. Additionally, a "What's new" tab may be shown on a minor version update (e.g. 3.0.3 to 3.0.4).
Some users have reported that these tabs open all the time. This article presents some possible solutions and workarounds.
You are running two different versions of Thunderbird using the same profile
If you are accessing the same profile from two different versions of Thunderbird, the "What's new" tab will be displayed each time you start Thunderbird.
Possible Solution: Ensure both instances of Thunderbird are updated to the same version.
An extension is causing a conflict in Thunderbird
Possible Solution: Try starting in Safe Mode Thunderbird. If that resolves it, the problem is being caused by one of your extensions. Click Tools > Add-ons on the Thunderbird menu to disable or delete extensions.
A preference is being overridden
Some people have a user.js file. This file might contain preferences that are not required, but make Thunderbird think it is starting up a new version each time.