Hello every body

I have an internet connection :

512 kbs
ISP provider is Shatel (I'm from Iran)
and my modem is TP-Link 8811 connected to my PC via LAN cable.

In connection status dialog box I see for example 500 kbs is recieving speed but when i start downloading something using firefox it deals most usually with speed around 10-20 kbs!

I have installed NetWorx to see all incidents related to monitor my transfers and something too interesting has happened:

In this software it started to count the amount of my transfer from the moment of running it, and after a quarter of hour it showed 50 mg received while my download bar just had received less than half of that amount! and it shows my transfer rate:

Current 42 kbs

average: 50 kbs

maximum: 102 kbs

(and while these amounts naturally change slightly)

but in firefox dialog box for download i see download transfer rate around 11-12 or a bit more or less!

i have tweaked all related registries of firefox or discarding the limitation applied by Microsoft bandwidth in registry.....but still that problem doesn't get solved.

PLEASE HELP!!!