Enabling the complete glass look requires more than a few steps—you'll need to install the Glasser extension, the Stylish extension, a number of user styles, and another extension to fix up the font colors. All of the instructions for setting up the glass theme can be found in the DeviantART guide, but the glass menus in the screenshot are thanks to the userstyles.org link sent in by reader Scott. Readers should note that the glass menus are a little buggy, and the overall style hides a lot of UI elements you might be used to, so while it's not for everybody it's an interesting customization worth a look for those serious about tweaking their browser.
You'll need to be running the latest beta of Firefox under Windows 7 or Vista to be able to use the glass menus, but you can install an older version of Glasser and use the rest of the glass effects. For more, check out All-Glass Firefox, or take a look at the more productive Aero Geek customization.