I don't know what the problem might be but I see a test matrix you should fill in.
What behavior do you get with Mobile IE, IE7/Windows and FF in both LAN and Internet, and both with and without IIRF installed?
That is 3 x 2 x 2 cases to test. 12 trials.
This might help you narrow down whether the difference in behavior you see is due to the browser (IE7/FF), the filter (IIRF/not), or the network (LAN/Internet), or some interaction among them.
You might also want to add in a few other computers to the mix - try IE7 on several different machines. And go visit someone's house to try IE7 from the internet.