Are you sure its not your Anti Virus program blocking the site, or Firefox being blocked itself?

Have you tried IE or Chrome and see if it loads amazon?

SEE the below articles too for instructions.

• Configure firewalls so that Firefox can access the Internet
• Websites don't load - troubleshoot and fix error messages
• Firefox can't load websites but other browsers can

Yep this is only in firefox. IE, SeaMonkey, Opera, and Safari have zero issue

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site. You mention already having tried that.

What do you get when you try to access Amazon.com. Blank page? Server not found? Certificate error?

Do you notice any notification icons in the address bar to the left of the address? You can click those for more information.

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

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
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• 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
• 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