You're right, the orthogonally-useless https://addons.mozilla.org/en-US/firefox/addon/6622/ is what I was remembering, it was bundled vaguely as "Developer Tools" in the installer until Firefox 3.
Obnoxiously hard to google for, I ended up having to dig up a Windows VM and an old Firefox installer to figure out what it was.
Re:Firebug Metamorphosis
by robin_reala on Wednesday October 27, 2010 @05:52AM
(#1837932)
Ah yeah, DOMi. It’s true that it’s seriously out of date now, but at the time (pre Firebug 0.4) and when combined with the Venkman JS debugger it was the only decent way to properly understand what was happening with problematic dynamic pages.
The Web Developer Toolbar was made by Chris Pederick, not Mozilla. It’s also never been bundled with Firefox.