When you use Flash Builder to launch a debug configuration for a web application in Firefox 4.0, the Flash Builder Debugger terminates.
To prevent Firefox from terminating plug-ins that it considers unresponsive, set dom.ipc.plugins.timeoutSecs to -1 in about:config. For more information, see Plugin hang detector.
This issue occurs because Firefox 3.6.4 and later terminate plug-ins that it considers unresponsive.
In Firefox, a plug-in hang detector determines when an out-of-process plug-in or plug-in script has stopped responding, and terminates the plug-in. The preference dom.ipc.plugins.timeoutSecs in the plug-in hand detector specifies the time period that must pass before it terminates the plug-in.