Created attachment 111887 [details] Screen recording of attempts to open FF menus Firefox Menus aren't appearing on click in Firefox 59.0.2+ and KDE 5.9+. I've added a screen recording...
A window manager is not involved in the context menus of applications. Please report to Firefox
Ok, sure I can do that. This does only happen in KDE however, and it occurs with both v59 and v60 (developer edition) of Firefox, leading me to believe it's down to a KDE change. I'll go ahead and add a bug there.
For posterity, here's the bug report I added for Firefox - https://bugzilla.mozilla.org/show_bug.cgi?id=1452402