I believe I was looking into the same mod a while back and did some research before I purchased. It also didn't work in Safari.
I contacted the mod owner at the time and he confirmed that there were browser related issues with the mod.
He didn't seem to worried about fixing them so I didn't purchase it.
Not sure if we're talking about the same guy but I don't think there are too many FB chat mods out there.
I bought it a while back and didnt see any comments about it not working on ie but i had it installed when i first bought it and it worked on ie, but not sure what i have done thats diff, and the install was done by someone else that i cant get in contact with anymore.