[Tracking Requested - why for this release]: Very visible UI regression that breaks primary UI. On my regular browsing profile, the back button is intermittently non-functional, even though it doesn't look disabled. Both the context menu and left-clicking it don't work. It persists per tab, that is, the button can be enabled in some other tab, and working, and switching back to another tab has it non-functional again. It seems like the button does have the disabled=true attribute set, but it looks enabled. OS X, 55 beta. Maybe related to bug 1362409?

adjusting state to reflect the fix in the duplicate bug