Because your server is not sending HTTP Expires headers, the browser is checking to see if what is in its cache is current.
The way it does this is to send the server a request saying what the date of what it has in the cache is, and the server is sending 304 status telling the client that what it has is current. In other words, the server is not sending the entire content again but instead sending just a short header to say the existing cache content is current.
What you probably need to fix, is to add Expires headers to what you are serving. Then you will see the NS_BINDING_ABORTED message change to (cache), meaning the browser is simply getting content out of its cache, knowing it has not yet expired.
I should add that, when you do a FireFox forced refresh, it assumes that you want to double-check what is in the cache, so it temporarily ignores Expires.