failed to launch while firefox is in use

Oh, and I quit out of firefox and everything went fine.

Agreed. Firefox can be upgraded while running, so I can remove this check. Also, giving a better error message would be fine, I'll look at this.
Thanks for your report!

Hmm, can you give some more detailed information about the error message please?
Usually, Apper should say something like "A program is running that has to be closed before the update can proceed. More information is available in the detailed report."
Anyway, I'll allow upgrading Firefox while it's running again. (FF will ask the user for a restart after upgrade anyway)

The Debian version now allows updating Firefox/Iceweasel while it's running.
Ubuntu will sync the changes sooner or later :)

This bug was fixed in the package packagekit - 0.7.4-2ubuntu1

---------------
packagekit (0.7.4-2ubuntu1) quantal; urgency=low

  * Merge from Debian unstable. Remaining Ubuntu changes:
    - debian/control: Have the libraries recommend python-aptdaemon.pkcompat
      over packagekit as we don't want the latter to be installed by default.

packagekit (0.7.4-2) unstable; urgency=low

  * Fixed qt2 library soname
  * Don't install GIR into multiarch path (Closes: #670374)
  * Updated packagekit-python description

packagekit (0.7.4-1) unstable; urgency=low

  * New upstream release: 0.7.4
    This is the ultimate-aptcc release, fixing nearly all known bugs!
    PK changes relevant to Debian:
     - packagekit-glib2: Add GType's for packagekit-glib2 enumerations
        (Stef Walter)
     - packagekit-qt2: Add simulateRepairSystem and repairSystem(bool) to
        the Transaction API (Daniel Nicoletti)
     - python: Speed up get_package_list (Tomáš Trnka)
     - apt: apt.cache.Cache() now reuses the dpkg of the chroot
        (Sebastian Heinlein)
     - apt: Fix modalias search on non-multi-arch systems
        (Sebastian Heinlein)
     - apt: Replace use of depracted python-apt 0.7.x API
        (Sebastian Heinlein)
     - aptcc: Add a new class to show cache opening progress
        (Daniel Nicoletti)
     - aptcc: Add AptCacheFile to handle all pkg cache openings
        (Daniel Nicoletti)
     - aptcc: Add SimulateRepairSystem and RepairSystem methods
        (Daniel Nicoletti) (LP: #496290)
     - aptcc: Avoid crashing when error messages containing invalid utf8
        is sent (Daniel Nicoletti)
     - aptcc: Change the way we mark auto-installed packages
        (Daniel Nicoletti)
     - aptcc: Correctly emit if a repo is enabled (Matthias Klumpp)
     - aptcc: Don't emit error if repo is empty (Matthias Klumpp)
     - aptcc: Emit newly-installed local package information (Matthias Klumpp)
     - aptcc: Emit packages that are untrusted with the
        PK_INFO_ENUM_UNTRUSTED enum (Daniel Nicoletti)
     - aptcc: Emit UnfinishedTransaction when we were not able to fix the
        cache (Daniel Nicoletti)
     - aptcc: Fix a multiarch bug that failed to resolve packages
        (Daniel Nicoletti)
     - aptcc: Fix crash when a package for an invalid version
        (Daniel Nicoletti)
     - aptcc: Fix resolving of gdebi packages when they have :arch field
        (Daniel Nicoletti)
     - aptcc: Implement support for InstallFiles()
        (Matthias Klumpp) (Closes: #606131)
     - aptcc: Implement SUPPORTED support (Matthias Klumpp)
     - aptcc: Improve the autoremove code (Daniel Nicoletti)
     - aptcc: Mark dependencies of a file-installation as automatic
        (Matthias Klumpp)
     - aptcc: Move ShowBroken to AptCacheFile, and added CheckDeps from
        apt-get (Daniel Nicoletti)
     - aptcc: Refactored to use AptCacheFile (Daniel Nicoletti)
     - aptcc: Reident the code and re-enforce KDElibs coding style
        (Daniel Nicoletti)
     - aptcc: Remove a code duplication and emit proper RestartRequired
       signal (Daniel Nicoletti)
     - aptcc: Reorganise parts of AptCC to make the code more readable
        (Matthias Kl...