Your post says you have Firefox 5. This information usually is listed under Help > About Firefox, or Help > Troubleshooting information.

Since you only need Firefox 3.6 for this one site, why not try using a portable version of Firefox 3.6.18? You can download it from here:

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

I appreciate your response. I'll try to figure out how to do that. I supposed that if 3.5x and 3.6x would work, version 5 would have to work at least as well, but I guess that may not be true.

Actually, that assumption isn't true in this case because the web site simply blocks users of newer versions of Firefox! There is a way to set Firefox to lie to a web site and say it's an earlier version, but given the nature of the site, I wouldn't want you to spend a long time filling in forms and end up not being able to complete the process and having to start again.

Thanks again. That explains it a bit more. The website said it limited browsers to only those that complied with standards that the worldwide organization that sets such standards has issued. I would have thought that since Firefox's older versions satisfied those standards, the newer ones would, too.

Firefox has always conformed to W3C standards, but the standards do change over time and that website may not be as up to date as Mozilla is with Firefox 4 - 5 - 6 versions.

Please click the Solved It button next to the answer that answered or solved your Firefox support issue, it appears when you are logged in, so this thread gets marked as Solved to help other users who may have this same problem.