The ongoing problems with Flash freezing in Firefox 3 after two seconds of playback resemble nothing less than a game of hot potato. Or perhaps watching two parents have a screaming match, with us regular ol' users playing the part of the sobbing kid that cries, "Why can't you two just get along?"
Adobe has updated the Flash 10 beta for Windows, Mac, and Linux, and with it comes the resurgence of the bug that installing the earlier beta fixed. There doesn't seem to be a given reason as to why the old beta is no longer viable, but it does work.
Install this latest version of Flash 10 beta the same way as before to solve the problem. Download the install file, close Firefox, uninstall the old version of Flash, install the new one, and then restart your browser.