By default, the Firefox button is off in Windows XP. To enable it, you need to hide the Menu bar (which contains File, Edit etc). Right-click in a blank area of the Navigation toolbar and remove the tick mark in front of Menu Bar item by clicking on it.

To hide the Firefox button and get the Menu Bar back, repeat the same steps to add a tick mark in front of Menu Bar item.

I don't want to give up the menu bar to gain the button. I was looking forward to the button where I could group my links, and I don't see that either. Frankly, FF4 is a disappointment. It loads a lot faster than the previous version but often when I click on a link I get an error message that the address is incorrect and 30 seconds later the site that I wanted opens. From my perspective, version 4 has unresolved bugs and I have been contemplating going back to 3.6 which was slower but worked flawlessly.

Sorry, you cannot have both the menu bar and the Firefox button. This behavior is by design as created by the Firefox developers.

I missed telling you how to get the group button. You will need to customize your toolbar and add the Tab Groups button to it. See the instructions in this article: Customize Firefox controls, buttons and toolbars

I would suggest you to post a new question regarding your problem with opening sites. We might be able to help you fix it.