Taken from http://tech.top21.de/techblog/20100624-debugging-flex-with-firefox-3-6-4.html;jsessionid=43F87175A24BAA1DF5B3920FCEC77E40
Firefox 4 tiene una nueva funcionalidad: matar plugins que no responden. Lo malo es que tambien mata tu sesion de Debug!
FIX:
Simply type about:config into the address line, then search fordom.ipc.plugins.timeoutSecs and change the integer value to 1000 or higher. That should do the trick.
