Firefox 3.5 has stopped receiving security updates and is not recommended for any system. Why do you want 3.5??

You can find obsolete and insecure releases of Firefox on the FTP server: ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/

Because my company does not support anything higher than 3.5 at this time. Our company's internal programs don't talk to anything higher than 3.5

Okay, if it's for internal use only, not for the web, that should be reasonably safe.