Sorry, my fault. I added another repository:
deb http://mozilla.debian.net/ experimental iceweasel-aurora
And this is my iceweasel version:
$ apt-cache policy iceweasel
iceweasel:
Installiert: 40.0~a2+20150527004004-1
Installationskandidat: 40.0~a2+20150527004004-1
Versionstabelle:
*** 40.0~a2+20150527004004-1 0
501 http://mozilla.debian.net/ experimental/iceweasel-aurora amd64 Packages
100 /var/lib/dpkg/status
38.0.1-5 0
500 http://http.debian.net/debian/ unstable/main amd64 Packages