Bugzilla

If it gets added to a new tab then it is not possible to symultaneously compose and keep an eye on emails.
Sometime you need other emails visible whilst composing.
Also I like being able to select to view an opened Write window from the Task Bar. 
An open Write window is easy to spot when needing to close down, so I can ensure it is saved as draft if incomplete prior to closing.
An open tab would get auto opened due to session data - assuming you do not have a product on computer that has a habit of deleting session data. 
Sometimes it is necessary to delete the session file if it has gone squiffy. 
It would mean you need to auto perform a draft save on any Write window that is open in a tab prior to shut down, sleep or close tab or close all tabs options. OR need to refuse to close tabs/all tabs or indeed Thunderbird until any Write tabs have been selected to be closed and the option for each individual tab has been selected to be saved as draft or ignored.

If it gets added to a new tab then it is not possible to symultaneously compose and keep an eye on emails.
Sometime you need other emails visible whilst composing.
Also I like being able to select to view an opened Write window from the Task Bar. 
An open Write window is easy to spot when needing to close down, so I can ensure it is saved as draft if incomplete prior to closing.
An open tab would get auto opened due to session data - assuming you do not have a product on computer that has a habit of deleting session data. 
Sometimes it is necessary to delete the session file if it has gone squiffy. 
It would mean you need to auto perform a draft save on any Write window that is open in a tab prior to shut down, sleep or close tab or close all tabs options. OR need to refuse to close tabs/all tabs or indeed Thunderbird until any Write tabs have been selected to be closed and the option for each individual tab has been selected to be saved as draft or ignored.

So you would also need the option to 'move tab to a new window'.