The implementation is straightforward for those running the latest Nightly version. Users can jump into the action by right-clicking an active tab or selecting two tabs at once. According to the official Firefox Nightly News report: "You can right click on a tab to add it to a split view, and from there select the other tab you'd like to view in the split. Or, multi-select 2 tabs with Ctrl/Cmd, and choose 'Open in Split View' from the tab context menu."
Sometimes, the best tech features are the ones you don't even actively think about - they're just there when you need 'em, quietly working on your behalf, without any fanfare or intensive effort required. Sometimes, though, those same sorts of scintillating slivers can have the unintended effect of being so seamlessly integrated into an app or process that you completely forget they're there and never get into the habit of actually using 'em. Or, worse yet, maybe you never even notice their arrival at all.