When there are major user interface changes you will always find some that like the changes and others who hate them. Personally, I prefer the new UI.

On Windows XP, it will default to the old UI and you need to hide the menu bar to show the new UI.

Firefox is highly customizable, you can set up Firefox 4 to look and behave similar to Firefox 3.6 if you do not like the new UI, as shown here - http://redherring2.wordpress.com/2011/01/17/firefox-4-restoring-firefox-3-ui

Thanks Tony - I am actually a brand new user to Firefox, so I first downloaded the 4.0 version, then cancelled it and installed the 3.6 after reading some comments... so I wasn't use to one, or the other but I did experience having problems with the backspace arrow (while on web pages) working...and haven't really been using it since (using Int. expl. still)  ?

I'm not happy with the new "Chro-mified" interface. The toolbars are smaller and so the icons are smaller too--everything seems jammed together. There's no reason on earth to have the interface be so crowded for a desktop environment.

• The "small icons" button does nothing.
• Selecting "Text and Icons" results in a horrible, unreadable mess.
• Gray on gray everything... how about some color?
• Firefox Mac UI designers: please take a look at iWork's Pages interface: well-designed icons, options to use large icons, plenty of space so that it's easy to scan...

In short, the new interface is unattractive and hard to use. I've been a loyal FF user for many years but I am very disappointed in this latest UI design.