The latest version of Firefox seems to have issues with high CPU usage in the plug-in container when running Flash videos. This is happening a lot as I have been reading threads along these lines. I have tried the same things that have been suggested in the threads. My sense is that you Mozilla folks know there is a problem and have not been forthcoming about it or what fixes you are working on. I suspect you have loaded the new version of Firefox with things that sap resources as has Adobe. What are you doing to fix the problem as you must know the problem cannot be fixed using your typical solutions.
Are you interested in providing more information on how this problem is affecting you? Discovering how to reliably reproduce the problem, either on particular SWF media or with particular hardware/add-on configuration, is the first step to a cure.
By the way, Firefox 27 is now available. No idea whether that will change this for you.
I am hesitant to upgrade as I believe my last upgrade might have at least contributed to the problem. Whenever I stream video, on Youtube or Facebook for example, my CPU usage shoots up to 100%. I have tried disabling add ons and extensions, uninstalling and reinstalling Firefox, trying an older version of SWF, resetting Firefox, running OS in safe mode, and lastly reformatting my hard drive and reinstalling my OS, which is XP professional. My computer is a bit old but I had no problem running programs utilizing SWF until about 6 weeks ago and have gone through multiple attempts to fix. I have AVG free, Malwarebytes, CC Cleaner and ZoneAlarm installed and run regular scans. No malware detected. I even added added RAM to the computer, cleaned off heat sink, fans, etc as I thought heat might be an issue. No better.
Could you try Firefox 25.0.1 on some of the same sites that give you the problem? You can use the "portable" version which installs into a single folder and doesn't interfere with your regular Firefox setup. I do think you need to exit regular Firefox before you can run portable Firefox, however.
Does that run better?
Did you try to disable hardware acceleration in the Flash player?
It is possible that your security software (firewall, anti-virus) blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.
Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.
I believe this is a Mozilla Firefox issue and is a common problem, based upon other thread comments. I am very disappointed with Mozilla as I suspect they have loaded too much bloatware on the application, as has Adobe Flash as evidenced by all the crap (McAfee) that automatically gets installed - which I immediately uninstalled. Hopefully there will soon be an alternative to Firefox. I am open to suggestions.