Today in "no good deed goes unpunished" - I regressed this in bug 1616008 because I missed the macOS-only rule at https://searchfox.org/mozilla-central/rev/a3266997560904b484fd89d6a6bcdeace4a13652/toolkit/themes/osx/global/menulist.css#15 , which takes precedence over the menulist rule in the prefs, so it sets min-height to 20px instead of 30px.

The :not() clause unhelpfully influences the specificity of the selector for menulists in general.
However, it is not necessary anyway, because xul.css overrides the min-height to 0 for [popuponly]
menulists using !important.

https://hg.mozilla.org/mozilla-central/rev/b3f37c4551b7

Can you please point me to a menulist where this change can be seen with the naked eye? I failed to notice any changes (post fix build - after fix build) after taking a look at some menus inside about:preferences.

This is a screenshot from mozregression --launch 2020-02-29.

And this is one from a more recent nightly. As you can see, the dropdown menus' heights now match that of the "advanced" button, and didn't in the "broken" build.

Thanks Gijs for the info. I could see the defect using old NIghtly from 2020-02-29. Verified that the dropdown lists like the one from Fonts, Zoom, Home, Search and others now respect the min-height of 30px on 75.0b7-build1 macOS 10.15.