How do I troubleshoot Flash Player's protected mode for Firefox?

Question

Starting with Flash Player 11.3, a protected mode was added as a new security enhancement for Firefox 4.0 and greater on Windows Vista and higher.  This security enhancement is designed to limit the impact of attacks launched from malicious Flash content (SWF files).  For additional details on this feature, please see Inside Flash Player Protected Mode for Firefox
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While we believe this feature is fully tested and reliable, if you encounter issues with Firefox we recommend the following troubleshooting steps.
Problem: Flash content does not appear, YouTube display errors such as "An error occurred.  Please try again later.", or Flash video will not function properly.
Solution: If RealPlayer is installed on your system, please disable the RealPlayer Browser Record plug-in.
Launch RealPlayer
Click the RealPlayer icon in the upper left corner of the window and choose Preferences.
Choose Download & Recording from the left hand panel
Uncheck "Enable Web Download & Recording for these installed browsers"
Click OK and close RealPlayer
Restart Firefox
If you are viewing this page in Firefox and Flash Player is not working, please click here for an HTML5 version of this video: YouTube HTML5 HD Link
Problem: RealPlayer is not installed, Flash Player crashes in Firefox
Solution:
Troubleshoot Extensions and Add-Ons by launching Firefox in Safe Mode
1. Exit Firefox
2. Click Start > Run
3. Type the following then press [Enter]:
     firefox.exe -p
4. Click Continue in Safe Mode
5. Attempt to reproduce the problem
If the problem does not occur in safe mode, please try to isolate it down to an individual plug-in.
1. Exit Firefox
2. Re-launch Firefox
3. Click Firefox > Add-Ons > Extensions
4. Click Disable on all of your extensions
5. Attempt to reproduce the problem, while enabling each extension until you can reproduce the issue
If the issue persists in Safe-Mode, please continue with the steps below:
Some users have indicated that random crashing stops when clearing their Firefox profile.  This step will allow you to temporarily test with a clean set of Firefox preferences, without losing your preferences.
Exit Firefox
Click Start > Run
Type the following then press [Enter]: firefox.exe -p
Click Create Profile
Click Next
In the Enter new profile name dialog, type "Test"
Click Finish
Select Test and click Start Firefox
Use Firefox as normal.  If the problem goes away, clearing your Firefox preferences, or migrating the important settings (bookmarks, etc) to a clean Profile may be a good permanent solution.
If the problem persists, switch back to your old profile by doing the following:
Exit Firefox
Click Start > Run
Type the following then press [enter]:firefox.exe –p
Choose your old account (typically "default")
Click Start Firefox
Solution: Try a clean Windows profile (You must be an Administrator to complete this step.  If your user account is not an admin type, skip to the next section for instructions on reporting a bug.)  Creating a new Windows user will rule out any account-specific preferences that may be enabled on your system. 
Create a user account in Vista
Create a user account in Windows 7
Use Flash Player normally.  If the problem goes away, this indicates a compatibility problem, either with software you have installed or with preferences set on your account.
Reporting a bug:
Please file bugs at bugbase.adobe.com.
Choose "Adobe Flash Player" as the product
Choose "Browser Integration" as the product area
Please provide as much information as possible with what you were doing before the crash and an exact URL where the crash occurred.
If available, please attach your Firefox crash logs to the bug using the Attachments field.
Show hidden files and folders. For more details, see the instructions for the operating system that you use: Vista and Windows 7.
Crash logs are located at C:\Users\username\AppData\Roaming\Mozilla\Firefox\Crash Reports\
Logs will be located in either the pending or submitted folder, depending on whether or not you submitted the crash report to Mozilla.
Last resort
Finally, if these problems are blocking you from using Flash Player we recommend, as a last resort, you either revert back to Flash Player 11.2 or temporarily disable Protected Mode.  We strongly recommend that Protected Mode is manually enabled once this issue is resolved.
To revert to a previous version of Flash Player, please see this FAQ: How do I revert to a previous version of Flash Player?
To disable Protected Mode, add the following line to your mms.cfg file located in:
Windows 32bit: C:\windows\system32\macromed\flash
Windows 64bit: C:\windows\syswow64\macromed\flash
ProtectedMode=0
If the mms.cfg file does not exist, create one using any standard text editor (e.g.. notepad)
Depending on your operating system, you might need to first save the mms.cfg file to a writable location (such as your documents or desktop folder) and then copy the file into the destination folder using Windows Explorer.
To re-enable Protected Mode, simply remove the line from the mms.cfg file.
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