From version 3.6.4 Firefox started to use plugin-container.exe as external container over browser plugins in general and flash player specifically

This to prevent browser crashes in case of plugin crash. The idea is nice, the implementation caused to common plugins instability.

In cases when plugin is "hard-working" (and Flash is good candidate), the communication between browser and plugin-container can be lost. In that cases FF counts till 45 and kills the plugin, sometimes it causes to FF freeze also.

There are 2 workarounds - return to version 3.6.3 or disable "plugin-container" feature in FF about:config (search for "ipc" string):

dom.ipc.plugins.enabled = true

dom.ipc.plugins.enabled.npctrl.dll = false

dom.ipc.plugins.enabled.npqtplugin.dll = false