I spent 30 minutes writing a trivial extension taking advantage of the fullZoom implementation of Firefox 3 to make a given area fit into the viewport. Example with the following page of the International Herald Tribune.

IHT page before applying Zoom It!

Let's say I care only about the article. Right-click on the first section of the article, select "Zoom It!" and done :

IHT page after applying Zoom It!

The XPI is available for Firefox 3 all platforms from Disruptive Innovations' web site. /* Enjoy ! */

Update: one en-US dtd file had a bug, please reload if you could not make it work. Sorry for that.