have you rebooted your computer? If no do so.

Now I'm even more confused! I rebooted and it worked--ONCE!

I opened message just fine, read it, closed it, tried to open another one, back to new tab only.

I have reinstalled Thunderbird. I get the same result as rebooting. ---I can open one message into a new window, but once I close that message, I can only open messages in "NEW TAB" not "NEW WINDOW"

Any ideas? Option for opening new messages is set on "NEW WINDOW"

Don't have this problem at home, am not aware of any significant differences between setups.

set Thunderbird to use tabs,,,, then change it back to a windows. I have seen occasions when the setting to user interface shows does not reflect the true state of affairs. Like it need to be reset to get the message.

Your could try restarting from the help menu with addons disabled. See if it is one of them causing issues. (quite possible)

It was add-ins--Not sure which one there were about eight that I saw no reason to have active so I deleted them. Now it works. The addins that i need apparently did not cause the problem as they are still active.

From the menu .... Click Tools / Options... then the "Advanced" icon then the "Reading & Display" tab. Under the section that says "Open messages in:" select "A new message window". I had this same problem and found that mine was set to "A new tab". Now I have tried it several times under different email accouts and it is working fine. Ctrl-W will do the same if you have the message selected, but I prefer my to open in a new window on a double click. I use the preview pane for review, but if I intend to do something with the message, I prefer it in a new window. I hope this helps.