I can't see any double spacing in a compact mode.

I don't see any compact mode settings. This is for my desktop pc, not a mobile device.

Compact mode workaround in Firefox

Note that Compact Mode only works for some cases in a bookmarks drop down list and not for menus. To include more/all cases where compact mode doesn't work you would have to use CSS code in userChrome.css. Compact Mode also affects the height of the Navigation Toolbar and the Tab Bar.

I doubt that Mozilla will backtrack on the menu spacing any more than the other bizarre changes in recent versions. I have been using userChrome.css code since version 89.