Install Firefox Portable 3.6.18 to your hard drive for that website, and continue to use 5.0 for everything else.

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

From a technical support, my comment won't help. From a political (yup, that's what I said) standpoint my comment should help.

I had the latest (as of Jan. 15, 2012 or so) version of Firefox on my iMac, running the most recent OS, when I telephoned FAFSA at the U.S. Department of Education in Wash., D.C. to report I had been told I had an incompatible browser, and to ask for help. After some discussion, the tech guy referred me to his supervisor.

I explained the incompatible browser problem, in detail to her. After a lot of back and forth, and some rigorous questioning on my part, she said that the US Dept. of Ed. is aware of the problem, that nationally they have had problems. She said that the Dept. of Education hasn't re-certified acceptable browsers for about two years or so. I was incredulous, and asked why the Department was subjecting probably thousands of people trying to fill out college financial aid applications to this sort of difficulty. She said they wete aware of this.

I then got a bit wise-ass. I said this was a news story, that I was a reporter (I am), and said that although I was a parent trying to help by kid, I was now going to write a story about this. She absolutely freaked out and hung up on me.

I went elsewhere than my home to get the FAFSA form completed.