Reduce bookmark menu spacing in Firefox 91

Some tips on how to get back some more density with your menus and booksmarks.

Edit about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true

Create directory and file %appdata%\Mozilla\Firefox\Profiles\XXX.default-release\chrome\userChrome.css

/* Bookmark and context menu spacing */
menupopup > menuitem, menupopup > menu {
    padding-block: 0px !important;
}

/* Options menu spacing */
:root {
    --arrowpanel-menuitem-padding: 0px 0px !important;
}

/* Disable rounded tabs */
.tab-background {
    border-radius: 0px 0px !important;
    margin-bottom: 0px !important;
}

Some of the usual older methods of changing this no longer works since Firefox 91, including

• Setting browser.compactmode.show to true, then changing Density in Menu, More Tools, Customize Toolbar.