perhaps you could show an image of what you see? (screen shot)

the pic doesn't really show the issue....the second window will not open....i am unable to compose or respond anything . i uninstalled thunderbird and reinstalled, but have the same issue. BTW.,....this problem is only on my windows 10 computer...on my windows xp comp i am not having the issue.

Select Help/Troubleshooting Information, click Open Folder to open the profile folder, close TB, delete or rename session.json and xulstore.json. Restart TB and see if the windows appear correctly.

thanks for trying....this turned out to not be a thunderbird issue at all.....STUPID ME!!!! i have dual VGA and i was using only one monitor.....and for some reason the second windows were opening on the monitor that was turned off.......when i turned it back on....i was able to find the missing window and all is back to normal!!!!

I have filed bug 152148 as I could not find the issue already listed.