After an update to Firefox 3.6.6 I've noticed that while debugging Flash it would crash and display "The Adobe Flash plugin has crashed". It turns out it's due to a new feature called "Hang Protection" that allows Flash to run in it's own sandbox and not take down the whole browser when/if it crashes. After 45 seconds the browser will close Flash because it thinks Flash has stopped responding.
Hang Protection can be turned off so debugging with Flex Builder / Flash Builder will work properly. Follow the directions at this url.
Firefox 4 requires a change to a different property. Use about:config to alter the behind the scenes settings and look for dom.ipc.plugins.timeoutSecs and set it to -1.