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 (188.8.131.52).
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.