This has been reported as a bug, mainly with OS X users:

https://support.mozilla.org/en-US/questions/1245998

https://bugzilla.mozilla.org/show_bug.cgi?id=1508153

Does the same thing happen if you use other methods to open a message, such as right-click, Open, or using the Enter key on the normal keyboard or the numeric keyboard? Does it happen in safe mode (hold Shift when you launch TB)?

Thanks for the reply. I should have mentioned that I am using Windows 10. I tried it in safe mode as well as removed all the extensions etc. NO luck. I was always able to right click on an email in the inbox and choose the option to open it in a new window (or click control O) to read the email (but same as before when I double click it in the inbox it still open it as a new email. (This only applies to the Inbox - If I double click an email in the Sent or Trash it opens in a new window the old fashion way just fine).

Any additional thoughts are welcomed.

Did you test the other ways of opening besides double-click?

Sorry for the delay. I have the options (when I right click) Open in new Tab, Open in new conversation, Open in new window but I guess it's years of habit of double clicking to open an email in a new window is what I really want and not a edit as new window.

The point of the tests is to see if your mouse is working properly when opening messages any other way but double-clicking. If double-click is the only faulty method, I suggest you try it with another mouse.