Yip
Yip is a Firefox extension that lets you receive notifications from web applications. It currently works on Meebo, Filttr, TweetTabs and other sites that support notifications. Yip supports many popular notification systems ― Growl on OS X, Growl for Windows and Snarl on Windows, libnotify/notify-osd on Linux/ Ubuntu. And don't worry if you haven't installed any of these, you'll still get the standard Firefox notifications.
Once you have it installed, click on one of the grey stars below to test out the notifications. Or if you prefer something more useful, login into Meebo, talk to your friend and wait for a notification (obviously, Meebo must not be your active tab at that time).
Not using Firefox? Growler does the same thing for Safari (Mac). Download Growler or learn more about it here.