Synced tabs in Firefox browser till now can be accessed from menu, sidebar and by visiting the about:sync-tabs page. The last one which hasn’t been maintained has been removed by Mozilla in latest Firefox 55 Nightly version. The about:sync-tabs page is dead now and shows the ‘ The address isn’t valid’ error page when you visit in Nightly.

On the other hand, Firefox team has provided an option in the sidebar (bookmarks, history and synced tabs) to move between the right and left side of the browser window, where initially a change forced for sidebar to stay on right, that has been backed out after receiving criticism.

Mozilla removes about:permissions from Firefox 45

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