https://addons.mozilla.org/en-US/firefox/addon/1419
That is a plugin that turns your firefox into IE (well as far as the website knows).
In terms of your problem, reinstalling IE7 would be a good place to start.
Make sure you have the latest version & that your computer is up to date with all of the new updates. I have worked on several vista machines with this problem and installing the updates seemed to fix it.
Cheers
Nick