This is a bit peculiar. Using the option to upgrade via Help | Check for updates in 3.6.16 and updating to 4.0, you get a different layout than when you download Firefox from Mozilla.com and install it manually.

For example, as you can see in the image, the buttons are square instead of round. You can change the background by using a Persona, or a theme, but I wasn't able to get those square buttons back until I uninstalled FF4, reloaded 3.6.16 and then used the above mentioned method to upgrade to 4.0

Firefox 4 doesn't change the look all that much, but adds features. I found that you can change the look yourself to make it look like they show it in the tutorials, or keep it familiar looking. If you upgraded as I did, Firefox 4 kept all my settings, bookmarks, add-ons, and themes...so it doesn't look all that different. Right click on a blank part of the menu bar and uncheck "Menu Bar." Does this help? If you want to revert, click on the new Firefox tab which appeared, then Options, and recheck menu bar.

Thanks so much!