Firefox and IE both limit the amout of resources a Flash applet can use. Safari (at least the Windows Beta) doesn’t seem to have that cap and Flash runs a lot faster in it.
Generally it is a good idea to optimize your game so that it can run even on older hardware. I develop on a 5 year-old laptop, because I’m too cheap to buy a new computer. But it has the upside that I can be pretty sure if it runs ok on my machine, it will run fine for most players.