The author has marked this style as obsolete.
As Daniel Einspanjer pointed out, this can be removed through the browser pref browser.urlbar.hideGoButton. I saw that pref, but thought it was from an extension I tried out. Using the browser pref, the Go button does appear briefly, but it will disappear.
This is code I put in my UserChrome.css file. It removes the annoying Go button from the URL bar. In previous versions, the Go button was separate, but for some reason, Mozilla combined it with the URL bar in this version.
There may be issues with using this CSS and certain themes. I tried Noia, and it didn't display the URL drop-down correctly.
This is really my first attempt at something this major, hope I didn't miss anything.
There is a hidden browser pref that can do this for you:
about:config -- browser.urlbar.hideGoButton = true
.ad {
/* Your ad can be here instead. 10K unique visitors for $5/day. Mail me. */
}