The problem is NOT just with Firefox or i would just use IE11 on my Windows 8.1 desktop. The player crashes on Every Browser,it IS an Adobe issue it however is NOT a Browser issue, so the essential answer is there is NO solution until the kingdom of Adobe gets around to fixing it or not, unless you know of another Program that will take the place of the Flash Player.
Latest version of Flash Player released is 220.127.116.11, Can you please try uninstalling Flash Player using Uninstaller from here [Uninstall Flash Player | Windows] and Install latest version of NPAPI Flash Player for Firefox from here [Installation problems | Flash Player | Windows].
If you are still facing similar crashes, to help us improving your Flash Player experience kindly file a bug at https://bugbase.adobe.com/ with all the possible details.
Sorry for the frustration.
Please work through the video troubleshooting guide, in particular, see the section on disabling hardware acceleration in Flash:
If the crashes go away with hardware acceleration disabled, you're running into video driver issues. Check to see if newer drivers are available. If not, shoot me your dxdiag (instructions are in the guide), and I'll try to escalate to the vendor from my end.
If the crashes do not go away, let me know, and I'll collect some crash reports for further investigation.