Copying the file "/var/lib/dpkg/alternatives/xulrunner" from another machine fixed the problem.
The file was deleted upon purging the package "xulrunner-1.9", while the package "xulrunner-1.9.1" was still installed.
Successfully re-installed "xulrunner-1.9" afterwards, which is required by "firefox".
What triggered the problem? Was it caused by installing "shiretoko" without removing "firefox"?
Copying the file "/var/lib/ dpkg/alternativ es/xulrunner" from another machine fixed the problem.
The file was deleted upon purging the package "xulrunner-1.9", while the package "xulrunner-1.9.1" was still installed.
Successfully re-installed "xulrunner-1.9" afterwards, which is required by "firefox".
What triggered the problem? Was it caused by installing "shiretoko" without removing "firefox"?