Compact mode only works for spacing bookmarks in some menus and the code for userChrome.css you posted has not(.in-menulist) and thus won't work for menus like in the screenshots you posted.

You can experiment with code like this in userChrome.css: Using :root[uidensity=compact] means that the code only works if density:compact mode is selected.

:root[uidensity=compact] menupopup :is(menu,menuitem) {
  padding-top: 0px !important; padding-bottom: 0px !important;
  min-height: 26px !important; /* adjust or use unset */
  height: 26px !important; /* you may not need this */
}
:root[uidensity=compact] {
  --arrowpanel-menuitem-margin: 0 4px !important;
  --arrowpanel-menuitem-padding-block: 2px !important; /*adjust*/
}

Since Fx 92 you need to add this line to your code.

  min-height: unset !important;

See the page where you got the code.

Thank you, much better now. I don't understand why FF do not has all this build-in into the "compact" mode.

• I set my height to 21px

The tabs still has top/bottom spaces, but it will do for now.

Thank you.