I have just installed MSIE 9 and Firefox 4 rc2 on my computer.
User interface on both browsers are similar. Firefox'es home-button moved to right of url bar. Just back-forward buttons are visible. Tabs are on the top. Url-bar are shorter.
Both systems support HTML5 and CSS3.
The visual difference between MSIE8 and 9 is more than the one between Firefox3 and 4.
Firefox let resize textareas.