See this https://support.mozilla.org/en-US/kb/keep-flash-up-to-date-and-troubleshoot-problems

Been there, done that. I have the most up-to-date Firefox and Flash Plugin. Quitting and restarting firefox helps for awhile but the problem recurs.

The support page says: If Flash continues to crash, updating to the latest version of Flash should fix the issue.

But I have the latest version.

Try to reset Firefox. https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems

You can check for problems with current Shockwave Flash plugin versions and try this:

• check for updates for your graphics drive drivers
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration
• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)
• disable hardware acceleration in the Flash plugin

See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

Try to set the Flash plug to click-to-play by setting the permissions for Flash to "Ask To Activate" in "Firefox/Tools > Add-ons > Plugins".
That way you can activate the Flash plugin when you need it via the Lego block icon on the location bar.

I was having this issue. I uninstalled FoxIt Reader (including the browser plugin that came with it). I installed Adobe Reader instead. Now when I go to a page with flash it works. YouTube is working for me now also.

Since I use Firefox both at work and home and it wasn't crashing at work I reasoned it had to be the FoxIt Reader plugin. It was the only thing that was different between my PCs.