All release versions of Firefox are available from ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/

If you just need it for one site, an alternative is to download the portable version of Firefox 2.0.0.20 from http://portableapps.com/apps/internet/firefox_portable (see the legacy versions near the bottom of that page)

When you install the portable version, select a location on your hard drive. This version will be totally independent of your current version of Firefox and can be used for that one site.

Maybe you only need to spoof the user agent if the server checks the user agent.

• User Agent Switcher: https://addons.mozilla.org/firefox/addon/59

• Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.8.1.20) Gecko/20081217 Firefox/2.0.0.20

There are other things that need attention:

Your above posted system details show outdated plugin(s) with known security and stability risks that you should update.

• Shockwave Flash 10.1 r85
• Java Plug-in 1.6.0_13 for Netscape Navigator (DLL Helper)

Update the Flash plugin to the latest version.

• http://www.adobe.com/software/flash/about/

Update the Java plugin to the latest version.

• http://www.oracle.com/technetwork/java/javase/downloads/index.html (Java Platform: Download JRE)