Firefox in its application .desktop file provides a couple of Desktop Actions ("jump list actions"), for opening a new tab or a new incognito tab. Since Plasma 5.6 those actions are also supported by KDE Plasma and there's no reason these actions should be restricted to Unity (OnlyShowIn=Unity).
They are currently shown in Plasma and work just fine as I did not implement support for OnlyShowIn/NotShowIn in Desktop Actions (which I didn't see explicitly mentioned in the spec though). For Plasma 5.8 which will have MPRIS controls in the context menu I want to add support for this to avoid double entries in media players (many abuse static Desktop Actions for player controls).
However, I don't want to break users relying on the ability to open new (incognito) tabs through the task bar in Plasma, hence I request the OnlyShowIn for Firefox desktop actions to be dropped. (Alternatively "KDE" could be added, but I don't see any reason for not enabling them globally).
Status changed to 'Confirmed' because the bug affects multiple users.