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.