Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0
Thanks for the bug report. I reproduced it on emacs-25. Also on Emacs
24.5, so it is not a regression. The bug appears to be fixed already in
the Emacs master branch, though, so I will (perhaps boldly) close the
report as already fixed.