Comment 3 for bug 1176490

At least for Thunderbird, there is a fix that has to be done every time you restart the computer:
1. Move your mouse to the launcher icons area (it can either be on one of the icons or in an empty area)
2. Hold down your mouse button and move up or down. Don't hold down too long without first moving the mouse; otherwise, you'll be moving one of the icons.
3. Let go.

I'm guessing this also applies to other applications.