From my perspective the Mozilla Foundation has done a terrific job developing Firefox and more importantly distributing and marketing the product. The latest version of Firefox is a very shiny software product and has been remarkably stable on the few computers I've installed it on. 

I haven't yet installed it on my main work computer; however, and tonight I was prompted to install the upgrade--not just an upgrade for Firefox 2, but upgrading to the completely overhauled Firefox 3.

Autoupdate notification mechanisms really are wonderful creatures for increasing the penetration of new versions of software. It is a damn shame that Microsoft didn't or couldn't manage to increase the market share of IE7 by harassing IE6 users until they upgraded.

One of the best things that Adobe has done is build in easy upgrade paths for its Flash software so that the vast majority of users are using the latest version of the software.