If I open the Image Manager in Firefox or IE on Windows, it comes up with no problem. If I open the Image Manager in Firefox on a Mac, the browser window is still grayed out, but the Image Manager window opens and pops behind the original window. If I click on the original window, it does move the Image Manager window to the front so I know the core functionality is there.
This also happens on your ASP demo site. I did notice that this problem does not occur on your .net demo site, if that helps any.
Thanks!