I'm not sure what your support policy is. There are some issues in IE7 if that matters to you. Screenshot of IE7 below.
There more significant ones are:
Yah this is because twitter is html 5. IE 7+8+9 don't support much html 5. Update your browser ;)
Update your browser.
Closing as outdated, IE 7-9 usage is trivially small at this point.