Quite simply, I just cannot see what should be font icons on lots of websites, regardless of if I use Firefox, Chrome, or IE. Here is a screenshot of what I see when I visit icomoon.io
I cleared the whole cache (everything from all time) in Firefox but it's still not displaying them. They did work in Chrome when I cleared the cache there though. They also don't show up in IE9 either after clearing that cache.
In Firefox, under Tools -> Content -> Fonts -> Advanced, I have the option checked for "Allow pages to choose their own fonts, instead of my settings above". Is there anything else in Firefox that I should check?