You'll need to start by opening About:Config to set some preferences. Type "about:config" in the URL bar and hit enter. If this is the first time you've done this, you'll be presented with a warning message telling you to be careful, and you should click the button saying you will.
Find the following prefs and set them the following prefs to them as listed. This will disable the built-in plugin hang monitoring, use the standard Flash sandboxing configuration, and disabling disable Firefix's Just-in-time JS compiler suite ("JITs ") for better debuggability:
* Set dom.ipc.plugins.hangUITimeoutSecs to 0