Comment 8 for bug 1226952

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=356250 has a patch which was apparently once applied to Ubuntu's firefox to fix this precise issue. The old bug is LP bug #31746. It was closed as Won't Fix for unclear reasons many years ago.

(I tried to link the Debian bug here as "also affects distribution ..." but Launchpad throws a timeout error for that.)