For reducing the height of notebook list items, refer to this thread: Dead area at the bottom of notebook list when trying to condense spacing using CSS
For reducing the height of note list items, apply this CSS to userchrome.css
with your desired item height (and item font height as needed):
.note-list-item-wrapper, .note-list-item {
height: 30px !important;
}
.note-list-item span {
font-size: 12px;
}
Reducing note list item height may cause dead space at the bottom of the list as described here: