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!