It's up to the site administrator to update their site to make it compatible with Firefox. Until then, you can use the following add-on if you still want to use 5.0.1:

http://coralietab.mozdev.org/

thank you, but it didn't work. Any other idea. If there is nothing else, how do I down load 3.5 or 3.6. can I have both on my lap top at the same time since I rarely have to access Fasa?

Are you trying to access this website? It does work for 5.0.1 which I am using:

http://www.fafsa.ed.gov/

It works until I click "start here"

Are you using IE 8? If you are, once you are at http://www.fafsa.ed.gov/, right click anywhere on the page and then choose "Using IE Tab" then "Use Internet Explorer", the website should open in IE and if you click on START HERE, it'll bring you to the log-in page.

Install Portable Firefox 3.6.x to your hard drive so you can access that website.
http://portableapps.com/apps/internet/firefox_portable/localization#legacy36

It won't affect your Firefox 5.0.1 installation at all, like installing a 2nd regular version of Firefox would.

Thank you! I installed the portable and it worked!