Firefox 4 nightly build debuts App Tabs
Firefox 4's App Tabs look pretty much like a pinned tab in Google Chrome. For now, an App Tab appears to simply force a tab to the left and display it with the favicon only. It's a great way to save additional space on your tab bar, and a feature I use religiously in Chrome.
Right now, you'll only find App Tabs in the Mozilla nightly builds of Firefox 4 -- which you can download here for Windows, Mac, and Linux. To create an App Tab, just right click and select the option from the context menu.