I had this problem, specifically with Zuma Blitz, but also noticed a bit of lag with other intensive flash based games and some flash video.

I tried a few suggested fixes to no avail so here is what I did. On a whim I remembered all the problems that were had in previous versions of Firefox with the flash plug-in container and I thought to myself surely this won't fix it.

Anyway I opened up a new tab and typed in about:config clicked "I'll Be Careful I Promise" on the warranty warning window.

In the address bar at the top of the list I typed in dom.ipc.plugins.enabled and set it to false by double clicking the item in the list.

This disables the flash plug in container.

I went back to Zuma Blitz and started a game and lo and behold it ran smooth as silk, no choppiness whatsoever.

Just to let you all know though that disabling this plugin also disables the crash protection that it was intended for however since I have been running FF4 I have never had a crash problem. If you are running a system with a low amount of memory, say 1GB or so you may experience crashing however if you are running this small amount of memory you will most likely experience crashing anyway by either the browser or the plug-in container depending on whether the container is enabled or disabled.

This worked beautifully for me, I hope it works for you as well.

Scratch that last reply, while disabling the plug-in container did fix the main part of the game play it caused issues with other parts of the game and with other flash games as well so I was forced to re-enable it.

Here is the deal though it has something to do with the way Firefox 4 renders flash. I do know that disabling the plug-in container made a huge difference in the main game play but caused issues in other parts of the game.

I know it is not a hardware issue as I have plenty of memory and dual core processor and a good video card and my PC is geared for graphic intensive performance.

It's not flash or java as I uninstalled both and did a clean install of both.

The thing I found interesting is that the cpu and memory performed virtually identical whether or not the plug-in container was enabled or disabled.

I would say that in reality the easiest fix for this issue would be to revert back to an earlier version of FF until a real fix is found and implemented otherwise I will keep hammering away at looking into the fix.

I have exactly the same problem, same symptoms with FF5 (right down to disabling plug-in container helping some things but breaking others), so clearly whatever the bug is, it's been carried forward to the new version. Shame.