As far as I can tell, Firebug is the only one that can edit HTML-code and text live as you type it. Very useful, if you're for example trying to see how text would fit in a container and add one character at a time.
In Chrome when you edit the HTML, you have to press TAB or ENTER to exit "edit mode", and see the changes on your page.
In Firebug you can also enter HTML-code right away. In Chrome, you have to right-click and choose "Edit HTML". Otherwise, it will show up like <b>bold</b>.
I really want to change to Chrome, since it seems to run faster, but the live editing is too important for me.