Rated 5 out of 5 stars

mail.tabs.drawInTitlebar=false seems to be what you search for?

This review is for a previous version of the add-on ( 

Sadly a theme cannot set this config, as it cannot deliver prefs nor inject scripts. Or is there a way I missed?However I use that config personally for now, I have a version ready with all other issues fixed :(

Edit: Fixed now, downloadable via changelog and after review.