It looks like the guts of what Matt Constantine put into the Firefox add-on is covered by the userscripts mentioned here:

http://zenhabits.net/2009/09/minimalist-gmail-how-to-get-rid...

I'd guess that the Zen Habits article came first and then the Firefox extension was made for an easy one-click installation.

Edit: I'm having trouble getting those userscripts to run in my Gmail Fluid app. Maybe there's some Greasemonkey-specific stuff going on in there...