It seems that the Flash 11.3 roll out has not gone smooth for Firefox users, quite the opposite. Flash 11.3 introduces the Flash sandbox, or protected mode, to Firefox but it seems that the security feature is causing problems in some cases. Add-ons and other plugins which enable users to record Flash videos are at least part of the problem.
Both Adobe and Mozilla are investigating the issue and there's no proper fix for all the problems just yet, but both companies have released several workarounds. The problem only affects Windows Vista and 7 users.
Upgrade to Firefox 13.0.1
Users are reporting that videos don't play and in some cases that there are even crashes. These should have been fixed with the latest Firefox 13.0.1 update, which you should definitely get, but the rest of the issues are still there.
You can take several steps if you are affected. If video doesn't play, the problem is most likely caused by another plugin or add-on.
RealPlayer Browser Record has been blocked by Mozilla, but there's an alternative solution
Mozilla has already blocked the RealPlayer Browser Record plugin since it interfered with Flash's normal operation. The plugin will be automatically blocked in Firefox, but you can re-enable it if you want to. If you're having problems, it's best to leave it disabled until Mozilla lifts the block.
Alternatively, you can disable the video downloading feature in the RealPlayer plugin. Launch RealPlayer, go to Preferences > Download & Recording and uncheck "Enable Web Download & Recording for these installed browsers" to do this.
Try disabling plugins until you find the culprit
If RealPlayer isn't the cause of your problems, you will have to pinpoint the problematic plugin on your own. You'll have to disable any that you have installed one by one. First, it's a good idea to start Firefox in Safe Mode and see if the problem is still there. Adobe has step-by-step instructions on how to do this.
Disable Protected Mode
If disabling plugins doesn't work, there is one last thing you can do, disable Protected Mode (the sandbox mode) in Flash 11.3. You have to edit a configuration file, mms.cfg, to do this. You'll find it at "C:\windows\system32\macromed\flash" on 32-bit Windows and "C:\windows\syswow64\macromed\flash" on 64-bit versions. Add "ProtectedMode=0" without the quotes in a new line.
Downgrade to Flash 10.3
Alternatively, though it is not recommended, you can downgrade Flash to a previous version. Mozilla points users towards Flash 11.2, but that's an unsupported version with known security vulnerabilities. The alternative is Flash 10.3, which is still supported for enterprise users.