All Javascript on site performs horribly Slow in Firefox 4 when transparent PNG's are present on the page

Firefox 4.0 on Mac should be using OpenGL for hardware acceleration, while FF on Windows uses Direct2D. Since the problem is only occuring on some Macs, could it have something to do with their graphic card chipset - perhaps compare the specifications of the offending Macs, and also try to disable hardware acceleration to see if problems go away?

I suggest you may have a some slighty corrupted PNGs.

Suggest you try the following: resave the PNGs out of photoshop. Save both PNG-24 and PNG-8. Test .

I have seen this before with PNGs that did not quite save correctly.

I tested the site on two macbooks pros one a core duo 2 and the other a core duo. Both ran fine the menu was very responsive even while the video was playing.

Assuming you have one the other steps listed in other answers here on the "suspect macs) ( ie created new accounts and tested ) then i think you need to look at what else makes those Macs different ? What version of OS X ? What model ( ie macbook vs Macbook pro vs Air etc )

Try creating a new Firefox profile so you can run it with no extensions, addons, etc. installed. It may be that some extension (AdBlock, GreaseMonkey, etc.) is conflicting with some of the scripts in your page somehow.

I just tested it in FF4 on my Mac (MacBook Pro, 10.6.7) and it loaded up quickly, so it may just be something in your setup.