Do the Firefox crashes occur anytime you view Flash content, or just certain sites? Does it occur when you visit this page:
Flash Player Version (http://adobe.ly/wNzNAu)
Could you please visit the "about:plugins" and "about:support" in Firefox's address bar and copy/paste the results into a forum reply.
Are you running any anti-virus, anti-malware, privacy or save flash video utilities? If so, what are their names and versions?
For IE, have you checked to see if you have ActiveX Filtering disabled and the add on enabled?
Thank you for the detailed info. Would you be willing to do a desktop sharing session with us so we can take a look at the crash and see if we can figure out what's going on? If so, please feel free to email me at firstname.lastname@example.org.
In the meantime, I'd recommend reverting to Flash Player 11.7 until we can get this resolved.