Perhaps something is blocking twitter.com on your network. Try using [http://technet.microsoft.com/en-us/library/ff961507%28WS.10%29.aspx tracert] command to see if that's the case.

[[Server Not Found - Troubleshoot connection problems]] might help too.

Clear the cache and the cookies from sites that cause problems. "Clear the Cache": *Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now" "Remove Cookies" from sites causing problems: *Tools > Options > Privacy > Cookies: "Show Cookies" *https://support.mozilla.org/kb/Error+loading+web+sites You can try to reset (power off/on) the router. ----- Do a malware check with some malware scanning programs on the Windows computer.<br /> You need to scan with all programs because each program detects different malware.<br /> Make sure that you update each program to get the latest version of their databases before doing a scan.<br /><br /> *http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware *http://www.superantispyware.com/ - SuperAntispyware *http://www.microsoft.com/security/scanner/en-us/default.aspx - Microsoft Safety Scanner *http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page *http://www.safer-networking.org/en/index.html - Spybot Search & Destroy You can also do a check for a rootkit infection with TDSSKiller. *http://support.kaspersky.com/viruses/solutions?qid=208280684 See also: *"Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked