Having the text-entry field resizing was alone worth the upgrade to FF4 for me. Right now, I can grab the corner of the text entry box that I'm typing this into and resize it to any size I want. Very very useful.
Let me strengthen that statement. Our company has a web-based internal bug filing system that I use many times per day. Its default text boxes are way too small, and so now I use this feature many times per day and it reduces headaches significantly. So much so, that now I consider FF4 indispensable in my daily work.