Sadly, this is a deficiency in IHG's design which is most obvious when using a slow Internet connection, downloading lots of sets, and downloading sets that require a second Internet access before getting through the "click here" button to the real image.
It is caused because IHG is operating in a "single threaded" mode, meaning it cannot do more than one thing at a time. When the worst conditions stack up it can take so much of FF's CPU cycles that it appears FF has frozen. If you look at your IHG download window you will see that it is indeed downloading images. Be patient and when all the images are downloaded FF will return to its usual responsiveness.