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 just updated the Flash 10 beta for Windows and Mac, 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.
Interestingly, some of the cases where the old Flash 10 beta would fail to fix the problem--like CNN--now work. Nevertheless, nothing would be more preferable than a permanent solution, but as Rafe adroitly points out, the culture clash between Mozilla's open-source and Adobe's closed doors is as likely to be a part of the problem as the code SNAFU itself.