There was a very capable javascript debugger as a mozilla plugin (don't think it was an addon), years before firebug/browsers dev tools had one. It don't remember the name, it was something with a V.
Firebug was neither the first for step-by-step JS debugging, nor for poking at the DOM. Those two things were within the purview of Venkman—the original Mozilla JS debugger—and the DOM Inspector—Firebug's predecessor created in 2001 by Joe Hewitt, an ex-Netscape/AOL employee and the original author of Firebug itself. Honza's timeline is off. I've left a note in the blog comments about this.
