Slow script dialog on Flash Player and Firefox

Not all sites use hardware accelerated video, and decoding video is CPU intensive.  Twitch didn't use hardware accelerated video until pretty recently, and I'm not sure that all content will be hardware accelerated.   My guess, since you're seeing high CPU usage in this instance, is that you're not seeing hardware accelerated content.  The slow script dialog appears when we detect that Flash Player is using too much CPU time.

If you go to about:crashes in Firefox, is it logging the crashes?  If so, the crash reports would be useful in giving you a definitive answer.

The reason that I specifically asked about 64-bit Firefox is that the sandbox architecture there will reduce the amount of interprocess communication and associated CPU overhead significantly, and it will solve the majority of the hangs. 

For context, back around 2009, Mozilla declined to write a native NPAPI sandbox to address emerging malware threats, so we built one and bolted on to the wrong side of the plug-in interface.  It's architecturally inefficient, and Flash and Firefox pass enough communication back and forth that it can overwhelm the operating system's ability to reliably deliver all of the inter-process messages, in order. 

What happens in practice, is that one side of the conversation ends up waiting for a response that never comes, or they just get something random back, and the communication between Firefox and Flash hangs. Firefox's automatic hang detection will notice this condition after a few seconds and will kill Flash Player.  This causes the majority of instances of the crashed plug-in dialog being displayed on Firefox.

When Mozilla revisited building a production quality 64-bit variant of Firefox over the last year or so, they also built a native NPAPI sandbox to go with it.  The 64-bit version of Firefox isn't the one that you get by default at this point in time, but you can manually choose it.  By replacing our sandbox with a native implementation, you get the same kind of protection, but the amount of IPC communication is reduced by a multiple, which solves the whole class of problems related to unreliable IPC communication.  You're going to get much better performance and reliability.

https://www.mozilla.org/en-US/firefox/all/

We're constantly hardening Flash Player against emerging threats, and those security mitigations generally don't come for free.  It's very possible that you were near the limit for the system but still had good performance with a previous version, but a reasonable incremental increase in overhead was enough to push this particular configuration over the edge.  We (and our partners) have extensive performance test suites, and we're not seeing an egregious change in performance between versions in the data.  It's ultimately very unlikely that we would walk a security change of this nature back without at least tying it to a very specific language-level problem.

Long story short, I think you're going to be much happier with Flash in 64-bit Firefox, and would highly recommend that you try that first.  It's low-effort and *much* better overall as far as being a host environment for Flash Player is concerned, and the sparse memory address space used by the 64-bit process gives you much better protection against memory attacks targeting the browser...

If you'd still like me to look at the crash dumps for 32-bit firefox, I'm happy to do that.

Please go to about:crashes in Firefox

Click on the first few crash reports to submit them to Mozilla

Copy and paste the resulting links here, and I'll take a look.