I have Firefox and Thunderbird running nearly all the time that I'm using my Windows (XP SP1) machine. When I select "Read mail" from Firefox's Tools menu, it will start up a new Thunderbird, so that I then have two. It should check to see if one is running, and restore it/bring-it-to-front, or the like. I fear this may have similar complications for "New Message". I would hope the existing Thunderbird will make a compose window, but fear a new one is instantiated to do this.
This creates a hassle with the MinimizeToTray extension. If a user in Firefox selects Tools->Read Mail, he would expect his minimized Thunderbird to restore. But it doesnt, it loads a second instance of Thunderbird. Like the description said, it would be easier to restore and bring to front an instance of Thunderbird if it's already running. We may be able to program a workaround in the extension, but it would seem fixing this bug is a better soltuion.
This bug no longer appears on my machine. Running TB 1.5 beta and FF 1.5 beta. Clicking on Tools-Read Mail does not launch a second instance.
this is also fixed for me - WFM XP TB version 1.6a1 (20050920), FF Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9a1) Gecko/20050925 Firefox/1.6a1 marking WFM