I just wanted to point out the similarity between the intended functionality of the Side View and the old Tile Tabs extension for Firefox prior to v57. The extension provided a far more convenient way of viewing web pages side by side. The two most important features (in my opinion) were:
Double-clicking on a tab would tile that tab and the adjacent one next to each other in the same window without having to fetch either page again. Double-clicking either tab again would un-tile them. This means that it was extremely convenient for quickly comparing two tabs at once. With Side View you have to right click and choose ‘Open in Sidebar’, or click Open Side View and choose the tab from a dropdown menu, both of which are clunkier.
Clicking within a tiled tab would make that the active tab, which means that the back/forward buttons and URL bar would apply to that tab. This doesn’t seem possible with the Side View.
I feel like the Side View would benefit from similar features to appeal to users who miss the quick workflow of the old Tile Tabs extension. Or better still, add support to Firefox for displaying multiple tabs in the same window so that the extension developer can bring back the old functionality!