If I click on a URL in Thunderbird I get the following message occasionally.
Firefox is already running, but is not responding. To open a new
window, you must first close the existing Firefox, or restart your system.

What causes this to occur? It is most annoying when you have several
Firefox tabs open.

I get the same message when BirdieSync is running under ActiveSync and
is trying to access Thunderbird. Only this time the culprit is Thunderbird.