Please try disabling hardware acceleration in Flash Player:

1. Go to this Adobe Flash Player Help page.
2. Right-click on the Flash Player logo on that page.
3. Click on Settings in the context menu. The Adobe Flash Player Settings screen will open.
4. Click on the icon at the bottom-left of the Adobe Flash Player Settings window to open the Display panel.
   
   The image "fpSettings1.PNG" does not exist.
   
5. Remove the check mark from Enable hardware acceleration.
6. Click Close to close the Adobe Flash Player Settings Window.
7. Restart Firefox.

Is this problem on a Mac laptop with a Retina display?

Hopefully you can access the above settings dialog to disabled hardware accceleration. Two of the bug reports in your post are linkified, and when I check those, they are happening in 3-4 seconds, so it may be hard to make the change suggested in the first reply on YouTube. What about on Adobe's Flash test page or privacy settings panel?

• https://www.adobe.com/software/flash/about/
• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager02.html

If those are not possible, according to the bug report linked in your crash reports, you may be able to make the change in one of these ways:

(1) Use an external display (if available)

(2) Temporarily disabling Retina (HiDPI) support in Firefox. To do that:

• Completely quit out of Firefox
• Use Get Info on Firefox and check the box for "Open in Low Resolution"
• Start Firefox normally and you should be able to turn off hardware acceleration in Flash
• Completely quit out of Firefox
• Use Get Info on Firefox and uncheck "Open in Low Resolution"

As a Windows user, I'm not able to test any of this myself.