If you close Firefox, then start Firefox in Safe Mode, does the problem still occur? For instructions on starting in Safe Mode, see Use Troubleshoot Mode in Firefox.

i did what you say but i have the same problem

I see that you or an extensions have made changes to a lot of network.http prefs including enabling http pipelining. It is usually not a good idea to make such changes and making such changes can have the reverse effect and slow down Firefox.

Try "Reset all user preferences to Firefox defaults" on the Safe mode start window - See http://kb.mozillazine.org/Resetting_preferences network.http.max-connections 48 network.http.max-connections-per-server 16 network.http.max-persistent-connections-per-proxy 16 network.http.max-persistent-connections-per-server 8 network.http.pipelining true network.http.pipelining.firstrequest true network.http.pipelining.maxrequests 8 network.http.proxy.pipelining true network.http.request.max-start-delay 0

i tried everything but i have the same problem.

Which security software (firewall) do you have?