Using the Glasser add-on will give the area around your URL bar the transparency like IE 7 and IE 8. If you have Vista, then it will appear black when maximized. In Windows 7, it appears transparent (duh).
if it says you have a newer version of firefox, then download Nightly tester tools
and then force the download of glasser.
Just open with firefox, and install normally!
Download and install both Glasser
With all of this implemented, your firefox should now look like this below when maximized. Otherwise, it's transparent.
Hope this worked for you!