Oh, cool. Should I make all my usual tabs into app tabs?
The tabs that will light up are the ones that update themselves. For example, when you receive a new email in Yahoo Mail, it will appear on the page by itself. When that happens, the tab lights up. Other sites that update themselves are Twitter and tumblr and certain news websites (I think), for example. Tabs that don't update themselves don't light up, but making them into app tabs still has benefits. They save space at the top of your screen, which is always nice, especially when you're using a laptop. And app tabs are remembered by Firefox, meaning that they'll be there right when you start up Firefox the next time. I don't know if you're in the habit of using tab groups, but if you are, they'll also automatically appear in each tab group.