You should upgrade to the latest version of Firefox for all internet use, except for that one particular website. There is no point in leaving yourself vulnerable by using an old version of Firefox because your school is too cheap (or too lazy) to upgrade to a newer version of BlackBoard Vista which supports a newer version of Firefox that is still supported by Mozilla with security fixes.

My advice is to install a legacy version of Firefox Portable to use only at that particular website, and use the latest version of Firefox for everything else you do on the internet. This is the last of the Firefox 3.5 versions.
http://portableapps.com/apps/internet/firefox_portable/localization#legacy35

It is not that the school is too cheap, it is the fact that you are changing versions of firefox too quick to allow the website ( use by several universities for on line learning) for them to approve your new versions. You are only hurting yourselves coming out with new versions so rapidly.

Most everyone here is a volunteer, we don't work for Mozilla. The latest versions of BlackBoard that came out last last summer do support the Firefox 3.6.x version, which is still supported by Mozilla. Have you verified which version of BbackBoard your school uses? Then you can use the latest version of Firefox that is certified for that version of BlackBoard.

http://www2.blackboard.com/tuneup/viewpage?name=tuneup_browser_tuneup_windows_firefox
Select the link for the version of WebCT Vista or Campus Edition (version info is usually at the bottom of each page, in the footer) that your school is using, then see which version of Firefox is compatible.