My pet peeves with FF4 is the open new tab thing has dissapeared next to the existing tab, I had to place the icon near the URL...
Also, secure websites don't show the 'lock' or change colour in the URL area to show it's a 'secure' website.... are FF being a bit anal over 'secure' websites?
Lastly, no more status bars at the bottom...
I hope FF5 will be better