[agent327]

I'm not convinced that you fully understood the point of this idea. The point is not just to just have any two web pages sitting next to each other (this is what your modifications do, right?), but more importantly the interaction between those pages.

I can already have two pages next to each other using Windows 10, by snapping two windows to the two sides of the screen. However, while that provides the correct visual shape, the user experience is completely different, as clicking a link in the left window will not open that page in the right window, replacing what's already there.

[thro1]

That's exactly how Load this bookmark in the sidebar works, you can have any document loaded into the sidebar, clicking any link on it will result in replacing document in the active tab (the only one visible).

https://mozilla.crowdicity.com/post/728459 says about Side by side browsing mode which I see as: with any two tabs, the content of the default active/content-primary tab will be replaced by link clicked on the other visible tab.