Today, after few weeks of Flex absent and after installation of Firefox 3, I wanted to debug one of my flex projects. First, there was an error “Flex Builder cannot locate the required debugger version of Flash Player. You might need to install the debugger version of Flash Player 9 or reinstall.” It’s rather normal situation after installation of the new browser, so I went to Adobe site and downloaded newest version. After installation there were no errors, but debug window in Flex Builder showed that Flex App was <terminated>. It seems that I’m not the only one who have this problem. Check this link to see the ticket for this bug. Untill it will be repaired, the only solution is to use other browser or to stay on Firefox 2.