Firefox extension Yip adds notification support for web applications, even adding native Growl support for Mac users.

To add notifications to your browser, install Yip and then test them with web-based applications such as Meebo or Filttr for interacting with Twitter. For MAC users, the addon will integrate with Growl, for Windows and Linux users, a standard Firefox notifications dialog will pop-up.

The really exciting feature, however, is the ability to create your own Greasemonkey scripts that enable notifications for your favorite website—you can already find scripts for Gmail, Facebook, Friendfeed and more—we tested the Meebo notifications that work right out of the box, but your results may vary on the user-contributed scripts. The author has written instructions on how to use the API, so if your favorite web application doesn't work yet, it might in the future.

[via lifehacker]