This sounds a bit like bug 188953. What does this command print for you?
python -c 'import apt; print apt.Cache()["firefox"].candidateOrigin'
Can you please attach your /etc/apt/sources.list ?
** Changed in: apport (Ubuntu)
Status: New => Incomplete
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apport refuses to report firefox crash - not genuine
https://bugs.launchpad.net/bugs/186280
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