Unlike other browsers Firefox allows users to test new experiments in its stable browser with a new program called as Test Pilot, which allows us to install and test new experimental developments they are working which will be added later to the stable version but not all experiments come to a stable version.
Try New Firefox Experiments
People at Firefox have developed some fun and interesting new features for Firefox through the program called Firefox Test Pilot. If you want to try it yourself, you just have to download the add-on in your stable version of Firefox from the Test Pilot website.
Side View allows you to see two tabs within a single window or tab, very useful especially when you want to view two products side by side and do a comparison between them.
If you want to see how it works, here is the demo video.