For me this is almost a daily occurance. Now it should stop crashing firefox for good. Here are the steps to take.
1. Go to the “Tools -> Options” menu
2. Select the “Downloads” tab
3. Open “View and Edit Actions…”
4. Enter “pdf” in the “Search” box
5. Select “Change Action…”
6. Change the selection from “Use this Plugin” to “Open them with the default application”, which should point to Adobe Acrobat, Adobe Acrobat Reader, or your favorite PDF viewer

Enjoy.

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