Your only option is probably to create a userChrome.css file with code linked below. That code will change spacing in all menus, not just the side bar.

https://www.userchrome.org/how-create-userchrome-css.html https://www.userchrome.org/firefox-89-styling-proton-ui.html#menuspacing

Do you mean the history in the sidebar?

• https://support.mozilla.org/en-US/kb/use-firefox-sidebar-access-bookmarks-history-synced

Add CSS code to the userChrome.css file in the chrome folder in the Firefox profile folder.

.sidebar-placesTree treechildren::-moz-tree-row { min-height:22px !important; }
.placesTree         treechildren::-moz-tree-row { min-height:22px !important; }