try closing using ALt+ F and selecting exit from the menu.

The solution given by Matt does not fix this problem. But, Matt, thanks for your suggestion.

A friend of mine said he experienced the same problem about a year ago. Said he ran across a Mozilla Support article that addressed this problem but can't remember the solution or the article in question. He stated that the article's question did not directly ask this specific problem but did address it and the article's solution fixed the problem of having 2 (or more) simultaneously opened instances of Thunderbird running and how to get back to having just one open and running when the Thunderbird desktop icon was clicked.

dspainter said

The solution given by Matt does not fix this problem. But, Matt, thanks for your suggestion. A friend of mine said he experienced the same problem about a year ago. Said he ran across a Mozilla Support article that addressed this problem but can't remember the solution or the article in question. He stated that the article's question did not directly ask this specific problem but did address it and the article's solution fixed the problem of having 2 (or more) simultaneously opened instances of Thunderbird running and how to get back to having just one open and running when the Thunderbird desktop icon was clicked.

My problem exactly. I can not remember the fix. It was to use the menu and not the X or vis versa as I recall it. or click on the close you see when you right click on the entry on the taskbar. It was so simple.

Yes, according to my friend, it was so simple, one of those things that you would say you would never forget.

I've gone through the "Tips and Tricks" section of the Mozilla Support Help Topics and didn't find any help. My friend says he thought he found the solution in a Knowledge Base type achieve, similar to what MS uses, but I can't find a Knowledge Base in Mozilla. I'm thinking there used to be, but perhaps that is what the "Help Topics" is now meant to be. If it is it is not very helpful! All the other Help Topics don't seem relevant.

Anyone else out there have any known solutions?

Maybe alt+F4?

The solution from Zenos (Maybe alt+F4?) fixed the problem! I opened and closed TB several times to make sure it worked, and each time it now only opens 1 instance of TB

Thank you Zenos

Like we said, It was simple, just couldn't remember.