Some sites on the web are hardcoded to work with the IE browser. If you come accross one of these sites, and just hate switching from Firefox to IE just for that site, then you're in luck, as there is now a FF extetion that opens up a tab in Firefox, but uses the IE engine so you can access those sites now without switching browsers.
The Firefox extention is called "IE Tab" and is available from
https://addons.mozilla.org/firefox/1419/