I agree with the spirit of what you’re saying, but they aren’t really discriminating against browsers at all. As far as I understand it, they pretty much have an

if (!adPageElement.isLoaded)
{
    showStupidPopup();
}

in there somewhere. It doesn’t rely on any nefarious browser implementation-specific extensions; everyone gets that same code and runs it. As for the 5 seconds thing, IIRC some FF configurations were triggering false positives, but I think it was patched. It does seem awfully convenient, and maybe they only patched it because they got caught, but they also must have been morons to think something that obvious wouldn’t be noticed immediately.