During my development of an app in MVC4 using Twitter Bootstrap 2.3.2, I encountered a problem with Glyphicons when testing in Firefox and IE.
In Firefox, instead of the icons displaying correctly, I saw the following image:
I double-checked the reference to the glyphicon image files and ensured they were correct.
Interestingly, when debugging in Chrome, the icons appeared perfectly fine (even after clearing my cache to rule out any issues).