I noticed Firefox (Iceweasel) 4.0b12 is in Debian experimental now so I decided to take it out for a spin. I moved my old .mozilla directory out of the way first to get a clean start without any extensions or configuration tweaks.
The Firefox extensions are the main reason that I'm considering switching back. In this area Firefox still blows away Chromium, notably in how well extensions are integrated into the browser. On Chromium, Ad Block Plus is still limited to hiding the ad after it was downloaded and NoScript is still impossible to implement. I was pleased to discover that both worked flawlessly on 4.0.
Overall I was a little disappointed with 4.0. For all the effort that has gone into it, it clearly isn't the Chromium killer I was hoping for. On the other hand, I'm becoming increasingly annoyed with Chromium's quirks lately, so I might still switch back to Firefox.