I upgraded the Firefox packages for Debian 8 (Jessie) and Debian 9 (Stretch) now and in both Releases Firefox can use the extensions again.
Thanks for your great work! Besides - Debian 8 is used for desktop use, because it is running with less problems. Like https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=926066 or https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=910494
