From the Firefox menu, go to Tools->Options. There, on the Applications tab, you will find a long list of applications/document types and what is supposed to be used to open each of those. It would look like this:
In there, like I've shown here, find the Adobe Acrobat documents and instead of "Adobe Acrobat (in Firefox)", select "Adobe Acrobat Reader", i.e. the program itself. If that does not show up by default, select "Other", and browse to find Adobe Acrobat Reader.
There you go - no more crashes! :)