E: The value 'xulrunner-21.0' is invalid for APT::Default-Release as such a release is not available in the sourcesA web search brought me to a post on the Linux Mint forum, where somebody had a similar problem. The advice given, which led to a solution, was to check the sources.list file.
I had a look in mine and found that as well as my Debian Testing sources, there was a Debian Jessie source (added during an update of Synaptic?).
Removing that line got apt-get working again. Seems to be a bug where apt-get thinks Jessie != Testing. Anybody seeing the same error, check