[ubuntu-za] CANNOT OPEN THUNDERBIRD?

[vincent]

I'm having a problem with Thunderbird in that I backed up my home
directory on the desktop and then restored it on my netbook. All works
fine and all files were restored.
However I keep getting a message advising me that Thunderbird is already
open & needs to be closed.
On the desktop Thunderbird works fine.
What have I done wrong & how can I fix it without a re-install?

Vince