Hi Marc,
firefox is not in jessie-backports, but in unstable. The package which you may find there in the future is firefox-esr, which is in testing atm. For desktop-installations it's IMO better to just run testing or unstable if you're somewhat knowledgeable. You know, they say there are three flavors of debian: rusty, stale and broken. Probably should add a fourth fossilized flavor ;)
> I have updated my /etc/apt/sources.list file and installed both pkg-mozilla-archive-keyring and debian-keyring.
so you installed from mozilla repo before and you certainly had the debian repo - what did you have to change?
Also: how did you attempt to install the keyrings? check: dpkg -l ${packagename} - ii in front for installed
If they're not installed, obtain the keyring deb via HTTPS and install: dpkg -i {packagename} (HTTPS because you cannot verify keyring integrity without trusting the keyring in the first place, default-HTTPS does not give you the same guarantees as secure-apt but at least an attacker has to acquire a validly signed cert.)
regards, Arian
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