Filing a bug is going to be the best method for us to track the issue and follow up with you.
I looked at Backyard Monsters with my Win7 x64 box, with Firefox 11 and Flash Player 11.3.300.233. I didn't see any performance issues.
First, update to the latest version on labs, and try again.
If the issue is still happening, I'm wondering if you're running into a hardware acceleration issue. We did lower the bar for GPUs with this release, and we may be using your GPU where we weren't previously.
You can figure this out pretty quickly by right-clicking on a SWF and choosing Settings. Click on the left-most tab, and uncheck Enable Hardware Acceleration.
If the issue goes away, it's GPU related.
First, check to see if there are updated GPU drivers available for your machine.
Once you're current, if you're still experiencing problems, we definitely want to know about them.
Launch a Command Prompt window, and type the following (and then hit enter):
Follow the prompts, and hit Save All Information
Please file a bug, and include the output of the DirectX Diagnostic Tool in your bug.
If the issue persists, I'll see if I can help you troubleshoot it further.