Have you got some kind of addon to download though such things do not mess with downloading but it can be point to check too. Having the latest version of firefox will also help and clear all cookies and if problem don't fade away then reset the browser or just re-install it.

http://kb.mozillazine.org/Issues_with_Firefox

http://kb.mozillazine.org/Issues_with_Firefox#Downloading_opening_or_saving_files

thanks for the answer, its been solve this morning, all i do is reset all the network setting using CMD (ipconfig, firewall rules, winsock) :D

That was very good work. Well Done.