Why does firefox block before downloading the document?
Firefox has it's own http proxy settings. IE and chrome take proxy settings from windows settings. Please check if there is some difference between your FF and IE settings.
Next thing you can do is creating new, fresh profile in FF (make sure there is no addons installed) and trying again.
try making your javascript calls asynchronous. My wild guess (with no codes provided that is the only thing possible) is that you have some javascript call that is blocking. try optimizing and debugging javascript, maybe that will help you see where did the blocking appear.