You could try contacting the site themselves, they may quickly make it compatible with Firefox 5 . While you are using Firefox 5 report the site as being a problem using

• Firefox button -> feedback -> and use the option to report a site and enter its URL from the location bar
• alternatively use link http://input.mozilla.com/en-US/feedback#sad
• I do not think it is possible to use these links from firefox 3.6 !!

To downgrade to Firefox 3.6 see installing a previous version of firefox note however this version will not be supported for long.

There is also a paid for add-on that may help, and is officially recommended https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/

• or again an official recommendation use another browser !

Install Firefox Portable 3.6.18 to use at that web site. It won't affect your current version of Firefox at all.

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

The portable version doesn't work on my Mac. So what now?

Consider downgrading to firefox 3.6, or if only a single site causing the problem, consider using an alternative browser for that site.

You would be better starting your own question, unless you are also talking about the FAFSA site, in which case have you tried

• contacting the site and asking about Firefox support.
• reporting the site as not working on Firefox, as mentioned in my earlier post

Portable Firefox versions for Mac OSX are available here:
http://www.freesmug.org/portableapps/firefox/

I have been having this problem too. I just tried using that portable Firefox and when I go to the FAFSA page it is asking for my full name. I don't remember ever having to put in my real name when logging in there, just the first two or three letters of my last name along with my SS # and birthdate. I'm concerned this may be a copycat site that is collecting personal info so won't be using. I'll wait to hear from the FAFSA people about what to do about this problem. I emailed them. Beware people.

If you are lucky maybe others who use the site may comment. Also note Firefox does have anti phishing features. What is the URL seen in your location bar ?
does it match that given in the Original Post (http://www.fafsa.ed.gov/index.htm )

• see also http://www.mozilla.com/en-US/firefox/phishing-protection/

Firefox has an excellent indicator for secure websites, you need to learn how to "read" what you see.
https://support.mozilla.com/en-US/kb/Site+Identity+Button

When the Site Identity button is blue, that indicates that the site's domain has been verified, and the connection between Firefox and the server is encrypted and therefore protected against eavesdroppers.