User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:56.0) Gecko/20100101 Firefox/56.0 Build ID: 20170625030210 Steps to reproduce: Sidebar extensions like Tab Center Redux which implement "Vertical Tabs" functionality currently can't use the tab (UI) context menu, forcing them to implement their own custom tab context menu, replicating standard Firefox functionality. There are a few problems with this approach: * Functionality might not work exactly the same as in the native tab context menu * Items will be missing ** If Firefox adds new context menu items the addon won't have those immediately ** There is no WebExtensions API for "Send to device", so addons can't replicate that functionalty in the tab context menu of their own UI (which is HTML) ** WebExtensions that add their own context menus to the "tab" context won't be shown in the replicated tab context menu. A way to show the tab context menu (some kind of API?) for HTML components in a sidebar extension is needed very badly. https://bugzilla.mozilla.org/show_bug.cgi?id=1367160 is related.
I think the bug 1280347 aims to solve same problem. I also believe that we should not implement sidebar context menu by own. Related issue on my "Tree Style Tab" addon: https://github.com/piroor/treestyletab/issues/1333
Another bug 1396031 seems including effective API proposal.