I have been having this issue in Ubuntu for a while. The problem occurs when I play Flash content (ie. YouTube or Google Video) and just as the video is about start playing Firefox crashes. This does not happen each and every time though, but its pretty regular.
So here is how I fixed the issue.
STEP 1: Download the latest Adobe Flash plugin from http://get.adobe.com/flashplayer/
STEP 2: Open Synaptic Package Manager (System > Administration > Synaptic Package Manager) and search for “flash”. In the list find the package called “flashplugin-nonfree” and mark for Complete Removal. Apply the changes and close Synaptic.
STEP 3: Go back to the Adobe Flash plugin you downloaded and install the package. Restart Firefox and your done!