apt-get -s install firefox
Reading Package Lists... Done
Building Dependency Tree... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or that some of the repositories
in use are in an inconsistent state at the moment.

Since you only requested a single operation it is extremely likely that
the package is simply not installable and a bug report against
that package should be filed.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
  firefox: Depends: xulrunner (>= 1.9-0.61.beta5.0blag.f9)
E: Broken packages