Hi

This is something that I understand we are still testing at present but hope to make available in an update very soon.

In the current Firefox 92 release you can only reduce the menu spacing via userChrome.css. In the next Firefox 93 release (current Beta/DE) they have added support to reduce the menu spacing via "Density: Compact" on the Customize page.

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Note that "Compact" is no longer present by default as a Density choice in Customize.