NIS 2012 slowing down downloadspeed FireFox 13?

Computer: 7iCPU, W7 Ult 64bit, 6 MB RAM, NIS 2012. I am using IE 9 as well as Firefox 13.02. Today I experienced a slow download speed on Firefox.  A speedtest showed the following results IE9 download 95 Mbps (out of max of 120 MB) and Firefox 40 MB. That used to be also 95 Mb. Uploadspeed in both cases 10 Mb.  Switching off firewall and scans in NIS didn’t have any effect on the downloadspeed of Firefox.  Safe Mode in FF had no effect, switchen off  plug ins, no effect. Even (de-)installing  FF with Revo  didn’t solve the problem.  Windows Safe Mode with network, the downloadspeed in FF went up to 80Mb. I wonder if NIS is slowing down the downloads in FF and if so what should/can I change in the settings to get the same speed as in IE9 (95 MB)??