@itisomegakaiI guess there were already many discussions about deprecation of features and nothing so far could stop Mozilla from moving in that direction. Complaining and/or offering proper reasons, why moves like that are bad/wrong, did not change anything in the past and won't either in the future. I gave up doing that.No idea what API to request. On Github, where they gathered info from developers, I wrote having ways to move/create/change/modify tabs, buttons, toolbars, content/items orders etc. would be great.My hopes are still very low and I don't believe Firefox 57 will offer something powerful and customizable (compared to what we have now) by then. I hope I'm wrong@streetwolfMozillas excuse for removing features is and always was something like "most users do not need/use that (see Chrome)". Additionally they want to have more control to reduce issue reports and browser/profile corruption.Looking at everything current add-ons can break once they get older and not updated constantly, I'm not surprised (see Foxtab for example).@marty60You are of course right, Fx ESR 52 will support XUL add-ons until Fx ESR 59(.2) replaces Fx ESR 52(.9) (~2018/Q2).