There have been issues on OS X where opening a message with the Enter or Return keystroke causes an Edit as New Message window to open, but I'm not aware of it happening on Linux. Does it happen if you open by double-click or the context menu? In safe mode (Help/Restart with Add-ons Disabled)?

I am using Linux (Lubuntu). I have checked safe mode and it also has the same thing and also does the same with double-click as well as via the context menu.,

I presume you have TB set to open messages in windows instead of tabs. The bug report for Macs might indicate when (version) this started. Otherwise, I suggest you file a bug report if an explanation doesn't appear.

Ctrl-O seems to open it correctly though I have found.