The sidebar button which you can add to Firefox toolbar or menu from customize dialog allows to quickly access Bookmarks, History and social features with one click. As a part -of upcoming UI change that’s going to land in Firefox 57 – Project Photon, Mozilla had moved the sidebar to the right side.
UPDATE: The change has been backed out by Mozilla after receiving criticism from users. Nightly now opens sidebar on left. Unless otherwise they change their mind, Firefox sidebar will continue to stay on left side.
Is this change necessary? Firefox team thought so. But it has definitely annoyed some Nightly users. As you all know, till now we used to access Firefox Sidebar on the left side. Mozilla hasn’t provided an about:config option to change thee sidebar position to left in Nightly, if you’re an advanced user, you can bring the sidebar to the left side of the window by adding the below code to Stylish or userchrome.css.
-moz-box-direction: reverse !important;