I think the idea that ‘Side View’ uses the side bar to display its content is seriously misguided. As is the mobile first feature; it’s an anti-feature!
A proper (multi) split screen view would be a great thing – in fact I came here through searching for this – but all splits should be at least treated equally. Generalize this model a bit, and you can easily think of three-way splits (or more!), horizontal and vertical splits, or combinations thereof, arranged in any way the user likes.
Do take inspiration from how tmux or vim work. For example, multiple vim windows can co-exist together with, say, NERDtree as a kind of side bar. tmux allows any number of splits, in any arrangement. The main difference is that in Firefox, each split could itself contain multiple tabs.
The side bar itself should always be a separate entity. (And no, having two possible side bars displayed, as mentioned elsewhere, is not a solution – still too restrictive.)