Here is an interesting website that lets you run old version of software in your own browser. The Browser Sandbox lets you run any browser instantly from the web with no install. It launches applications in an isolated virtual machine using a small browser plugin. It allows you run current and earlier version of many apps including Firefox, Foxit Reader, Gimp, Notepad++, OpenOffice, Skype, TweetDeck, VLC Media Player, etc.

I did try it out and was able to run Firefox 2 in my Internet Explorer 9.

One note of caution! When you agree to install the browser add-on, it will even install a Spoon Sandbox Manager. While you can always disable/uninstall the add-on via your briowsers add-on manager and uninstall the Spoon Sandbox Manager via your Control Panel, it is not so easy to uninstall the latter. You may have to exit the app via its Notification Area icon, use task manager to kill the Spoon process and even exit the iexplore.exe, before you will be able to uninstall it.