You can get this effect just by loading some document into the sidebar e.g. Show all bookmarks -> check Load this bookmark in the sidebar -> click that bookmark - I'm not sure how to do this easy in newer Firefox versions - with Browser Console maybe ?
If you want full Side by side browsing mode:
Some part of it is extremely easy - to display ALL tabs side by side:
For the other part - to have only two panels, sibling is easy, otherwise you would need some JS to mark a tab as something like content-secondary and handle it (e.g. there was an extension last_selected_tab doing like this AFAIR) - as you have only:
(Just tested in Firefox 60.4.0.esr - newer versions may need some minor adjustements in CSS, toolkit.legacyUserProfileCustomizations.stylesheets preference set to true and probably some other fixes. In 2008 I had a Norton Commander 'clone' made in such incredibly simple way, ALL file and media formats preview (plugins), with a bit of custom code for nsIProtocolHandler, and nsIURIFixup fixes :) , kudos to archView extension by Pike/Solar Flare.)
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.
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.
If you want full Side by side browsing mode:
Some part of it is extremely easy - to display ALL tabs side by side:
For the other part - to have only two panels, sibling is easy, otherwise you would need some JS to mark a tab as something like content-secondary and handle it (e.g. there was an extension last_selected_tab doing like this AFAIR) - as you have only: (Just tested in Firefox 60.4.0.esr - newer versions may need some minor adjustements in CSS, toolkit.legacyUserProfileCustomizations.stylesheets preference set to true and probably some other fixes. In 2008 I had a Norton Commander 'clone' made in such incredibly simple way, ALL file and media formats preview (plugins), with a bit of custom code for nsIProtocolHandler, and nsIURIFixup fixes :) , kudos to archView extension by Pike/Solar Flare.)R.I.P. Firefox.