Many times I see a replacement icon with "E0' stacked on top of "07" rather than font symbols in FFox. This happens frequently on codepen.
Today, I noticed it on http://www.sitepoint.com/store/the-ultimate-css-reference/ and finally decided to get to the root of the problem.
When I visit that page in FFox, each star in the 'Customer Reviews' section of the page is displayed with the replacement icon . When I view that page in IE or Chrome, I see the correct font symbol, which is a star. The replacement icon appears to be an image, but when I RMB over it or try to select it, it appears to be invisible to the mouse.
I assume that this is a common occurrence for FFox users - especially those who don't know how to fix it (if there is a fix).