STEPS TO REPRODUCE
1. start Wayland session
2. make sure Firefox is using CSD and the menu bar of Firefox is enabled
3. open Firefox CSD
4. maximize Firefox window (if Firefox window is already maximized on opening, unmaximize it and maximize it again)
5. right-click or click on something above the webpage, like minimize/maximize/close buttons in the upper right corner, url field, the hamburger button, "+" (new tab) button, entries of the menu bar (file, edit, view, etc), "x" button of a tab, etc.

OBSERVED RESULT
random and extremely annoying problems: sometimes nothing happens when I click on some entries of the menu bar, sometimes nothing happens when I click on minimize button, sometimes hamburger menu opens when I click on restore/unmaximize button, sometimes I can't close Firefox window by click on "X" button in the upper right corner, sometimes hamburger menu does not open or opens but does not close when I click on the hamburger button again, sometimes nothing happens when I right-click inside the url field, sometimes hamburger menu opens when I click on "+" (new tab) button, sometimes nothing happens when I click on "x" button of a tab, sometimes Firefox window is minimized when I click on "restore" button.

All these problems disappear after I minimize and unminimize Firefox window.

EXPECTED RESULT
mouse input should always work correctly.

SOFTWARE/OS VERSIONS
Firefox 76
Operating System: KDE neon Unstable Edition
KDE Plasma Version: 5.18.80
KDE Frameworks Version: 5.70.0
Qt Version: 5.14.2