Try using a Firefox Portable version installed to your hard drive. It won't affect your "official" version of Firefox, as it installs into its' own folderset and will use its' own Profile.

http://portableapps.com/apps/internet/firefox_portable/localization#legacy36

I'm having the same problem as tj. The link you gave gave a exe file. Is there any file I can get for Mac to do the same thing?

No sorry as those portable builds are for Windows only.

If you want Firefox 3.6.18 then see http://www.mozilla.com/en-US/firefox/all-older.html

You could also try fooling the site in thinking you are using Firefox 3.6.18 by using this Extension. https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/

Portable Apps doesn't have a version for Mac OSX, but there are MacOSX versions of Firefox Portable available at SourceForge:
http://sourceforge.net/projects/osxportableapps/files/Portable%20Firefox%20OS%20X/3.6%20r4.1/