Side View for Firefox
Ever wanted to watch YouTube videos while shopping, or keep up to date on Twitter while working on a spreadsheet? Side View lets you send websites and links to your Firefox sidebar to make multitasking easier than ever.
Source: Firefox Test Pilot - Side View
Sure, you could open two windows, but in some ways this makes it easier to manage. I’m a little thrown by how clicking links works though. Feels like they should be loaded in the sidebar.