I am using the KDEs app-menu. In order to use it with Firefox and Thunderbird, the addon global menu bar integration has to be activated which I believe has been initially created for Ubuntu/Unity. Unfortunately using this breaks all kind of context menus, pop up menus, like e.g. urlcompletion or the showing of passwords in the Firefox password manager.
It can be seen that these menus all actually appear, but only for a fraction of a second, before they disappear again. I believe this bug has been there for many months, I guess from the beginning of KDE-appmenu. The problem was, I could not reproduce it at all for a long time. In Firefox it does not always happen from the beginning after starting Firefox, but after a while of running it. Now that I activated the glibal menu integration in Thunderbird (where the bug seems to be there from the start), I could determine this connection.
What I can not determine if this bug is related to KDE-appmenu itself or is a problem of the global app menu or the interaction of both. Therefor I have also reported it to the global app menu integration. Unfortunately without a link or bug number, I just used "report issue" in Firefox/Thunderbird where it appears this is reported directly to the developer.

Reproducible: Always