Add possibility to have a left & right-sidebar (together). E.g. on a wide-screen to have TabCenter in one sidebar (e.g. right) and a other sidebar on the left side. https://testpilot.firefox.com/experiments/tab-center
In general it should be possible to have multiple sidebars open, no matter what side. The user has to decide which one should be open and what can be closed. Why limiting to one or two?
My suggestion would be to have a sub-menu for "sidebar left" & "sidebar right". (See screenshot as attachment with just "sidebar".)
Created attachment 8821843 [details] 2016-12-26 03_23_57-1325763 – Add possibility to have a left & right-sidebar (together) - Nightly (B.png