It was a fork off the Netscape Navigator which included a news reader, an email client, a browser and a kitchen sink, from what I remember. They took the browser part out and created Firefox, but it was called something else at first. Firebird maybe? Can't recall.
Maybe they mean lean compared to internet explorer toolbars? But yeah, it's never been minimal. And I doubt this would really add that much bloat memory-wise while running.
Mozilla Application Suite contained an email client and a HTML editor, among other things. Firefox was supposed to "just a browser", so to speak:
Firefox was created in 2002 under the code name "Phoenix" by members of the Mozilla community who desired a standalone browser rather than the Mozilla Application Suite bundle.