Did you install an extension from the Mozilla Add-ons site or from a third party site? To see whether one of those might be involved, you could try one of the following:

(1) Manually Disable ALL nonessential and/or unrecognized extensions on the following screen. If in doubt, disable:

orange Firefox button (or Tools menu) > Add-ons > Extensions category

Then restart Firefox using the handy links provided for that purpose and see whether you can load Twitter. Try Ctrl+Shift+r to bypass cached "failure" pages.

(2) Test in Firefox's Safe Mode

That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

Any difference with Twitter? Try Ctrl+Shift+r to bypass cached "failure" pages.

From the systemwide perspective, are you familiar with the Windows hosts file? This is a file that assigns alternate server addresses and can be used to block access to sites or even impersonate them. For information on how to check and/or clear out your hosts file, try this page: http://support.microsoft.com/kb/972034. I suggest reading through the "fix it myself" section before using any automated "fixits" from the Microsoft support site, so you can get a better idea of what is going to happen and also in case you want to make a backup of the file before it's zapped.

Finally, you may want to run some supplemental scans. These three tools are highly regarded (and free):

• Malwarebytes Anti-malware : http://www.malwarebytes.org/products/malwarebytes_free
• SUPERAntiSpyware : http://www.superantispyware.com/
• AdwCleaner : http://www.bleepingcomputer.com/download/adwcleaner/ (ignore banner ads for other products)

Do a malware check with some malware scanning programs on the Windows computer.
Please scan with all programs because each program detects different malware.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender: Home Page:
  http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked
• http://kb.mozillazine.org/Error_loading_websites

Hello jscher2000 & cor-el,

Thanks for your response. I have tried manually disabling the extensions and tested in safe mode and the problem still persists..

I wanted to try Malware Bytes Antispyware and guess what ? the website is not coming up either. Same message as twitter so now I am unto SuperAntispyware and hope it works........ so frustrating... thanks for your help so far

If you can't load the websites of anti-virus software then that is a strong indication of a possible virus infection.

Can you download those programs via another computer and save them to an USB stick?

Be sure to also download the latest virus signature databases.

If using various malware scanners does not fix it or if you are blocked from installing those scanners then ask advice at a forum that specializes in malware removal.

• http://forums.majorgeeks.com/index.php - MajorGeeks Support Forums
• http://www.security-forums.com/index.php - WindowSecurity.com Security Forums
• http://www.bleepingcomputer.com/forums - Bleeping Computer Forums

Did you check for the hosts file?

If you aren't able to access Microsoft.com, the instructions are:

"To reset the Hosts file back to the default, follow these steps:

1. Click Start, click Run, type %systemroot%\system32\drivers\etc, and then click OK.
2. Rename the Hosts file to Hosts.old.
3. Create a new default hosts file. To do this, follow these steps:
   a. Right-click an open space in the %WinDir%\System32\Drivers\Etc folder, point to New, click Text Document, type hosts, and then press Enter.
   b. Click Yes to confirm that the file name extension will not be .txt.
   c. Open the new Hosts file in a text editor. For example, open the file in Notepad.
   d. Copy the following text to the file:
   
   For Windows XP or for Windows Server 2003

   # Copyright (c) 1993-1999 Microsoft Corp.
   #
   # This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
   #
   # This file contains the mappings of IP addresses to host names. Each
   # entry should be kept on an individual line. The IP address should
   # be placed in the first column followed by the corresponding host name.
   # The IP address and the host name should be separated by at least one
   # space.
   #
   # Additionally, comments (such as these) may be inserted on individual
   # lines or following the machine name denoted by a '#' symbol.
   #
   # For example:
   #
   #      102.54.94.97     rhino.acme.com          # source server
   #       38.25.63.10     x.acme.com              # x client host
   
   127.0.0.1       localhost

   
   e. Save and then close the file.

Thank you so much jscher2000 & cor-el for your suggestions. The only thing I could download was superantispyware and I ran a scan with it and found nothing. Now 16hours later, I can log on to www.twitter.com and all the other sites like www.malwarebytes.org that I could not log on to yesterday. I have no idea what went wrong and how it was resolved but thanks for your help.