If we want to really get pedantic, Mosaic was a "first" -- the first WWW browser to support embedding inline images in Web pages. Previous browsers were text-only; you could post images, but they could only be linked to, not displayed directly inside a page. That made the WWW too similar to Gopher (http://en.wikipedia.org/wiki/Gopher_%28protocol%29) to really break out in popularity. Mosaic's support for inline graphics is what caused the Web's first real wave of general interest outside academia.
An application can have a GUI without being graphical. I'll confess to never having used WorldWideWeb on a NeXT box, though vids suggest it didn't do much in the way of typesetting or graphical rendering.
More here: http://www.wired.com/thisdayintech/2010/04/0422mosaic-web-br...