If Flash is up to date, try disabling hardware acceleration, first in Flash (you can find this setting in your Adobe Flash Player Settings), then in the Firefox browser itself. If so, you will need to manually install the latest version of Flash from the Adobe website. To check, go to Mozilla's Plugin Check page and see if Flash needs to be updated. In Firefox, the most common reason for a Flash plugin crash is an outdated version of the Flash player. Type (you guessed it) 'chrome://extensions' into your browser address bar and go down the list one by one to see if disabling one of the extensions resolves the issue (source: Spector]. You can also check to see if a browser extension such as ad-blocking software may be causing a conflict. To check this, type 'chrome://conflicts' into your address bar, and Chrome will display a list of any third-party applications known to conflict with Chrome.

If duplicate Flash files aren't the source of your problem, you may have software or malware on your system that's causing the crash.

Firefox flash plugin crashing