So far my favorite discovery in Firefox 4 is the App Tabs –

Each of the little red arrows point up to a “pinned tab” or a “app tab” residing to the left of the full tabs, and remaining there after Firefox has been closed and reopened. I can quickly and easily open my calendar, my mail, my Facebook, one of my wikis or the Hunger Site I like to click on every day.

I add an app tab by right clicking (or 2 finger clicking) on the tab I want to “pin”, like so,

and then clicking on “Pin as App Tab”. Couldn’t be easier!