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.
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.
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.