One of the major features of the newly released Firefox 3 is the support for offline applications (implemented as part of it's implementation of HMTL 5). For web developers this means that their users can continue to work on a application even when they are not connected to the internet.
Before Firefox 3, this was already possible using Google gears. Even today a ever increasing number of Google applications like Google reader, Google Docs and non-google ones like Zoho Writer, etc are already using this to provide offline support. With its current release Firefox makes this add-on redundant.
Meanwhile, for web developers who want to give this a try, here'sthe article from IBM developerworks that tells you how - Get Ready for Firefox 3.0. For those who are eager to see a working demo, check out the one on the article author's blog.