Huey quoted How the Internet Happened by Brian McCullough
The first genuine advertisement on the World Wide Web was published by Global Network Navigator, which in 1993, sold an ad to a Silicon Valley law firm, Heller, Ehrman, White & McAuliffe.
— How the Internet Happened by Brian McCullough (Page 5)
Must have been a good ad because the firm is still around (www.hellerehrman.com)
Sadly I can't seem to find a screenshot of the advertisement — I'm quite curious as to what it looks like.