I have done the troubleshooting suggested by Mozilla https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding
Firefox works fine but as soon as any Flash content is started Firefox locks up and I have to close the process in Windows Task Manager.
I can't go into Flash settings and disable hardware acceleration because Flash freezes Firefox completely before that.
Could you try disabling hardware acceleration via your system's mms.cfg file?
This will be located in your C:\windows\syswow64\macromed\flash folder (assuming that you are using a 64 bit version of Windows). Simply open up the mms.cfg file with a text editor and add the following line:
Save the file and try again.
If that fails, I'd recommend reverting back to Flash Player 11.7 until this can be resolved.
This didn't work for me. I had no file named "mms.cfg" in my C:\windows\syswow64\macromed\flash folder. I created one and added the entry, but it didn't seem to work after I rebooted the system. I confirmed by going into the settings and it was still enabled.