Sorry. I can not see what you are seeing since I have no access to a mac.
The code you have insert for the iframe is normal and has no faults.
But the more scripts you run on/in the page, the more likely that one will impact another negatively. Either you have two pieces of code that interfer with each other (the mootools comes with a bgiframe as part of the code and you have inserted an iframe), OR your z-index should be adjusted to force the iframe onto a different level. This sometimes stops behavior we do not want.