Hrm... my apologizes, I stand corrected then. I started getting into web development after Netscape was, for all intents and purposes, dead and IE6 was king.
Sure, but Netscape Composer was a WYSIWYG editor as well, not a web page inspector/debugger like Firebug. Honestly, there was not much to inspect or debug in web pages at the time...
http://www.seamonkey-project.org/