Firefox is very customizable, probably the only browser allowing so much tweaking of UI.
The omni.ja (which is actually a zip archive) file in "browser" folder (not the omni.ja in root folder) of Firefox contains all the styling and javascript files.
Some of following CSS files may need adjustments.

chrome/browser/skin/classic/browser/urlbarView.css
chrome/browser/skin/classic/browser/tabbrowser/tabs.css
chrome/browser/skin/classic/browser/browser-shared.css
chrome/browser/skin/classic/browser/searchbar.css
chrome/browser/skin/classic/browser/preferences/preferences.css

urlbarView.css has the styles for the address bar and drop down sites list.
tabs.css does the tab styling.
searchbar.css takes care of the search engine list / bar.
browser-shared and preferences might need some minor changes.

By changing the tabs.css, i got the tabs height of only 8px. In widescreen monitors, vertical space is much less than horizontal, so reducing tabs bar height gives more space for website.
Similar changes to urlbarView allows for a smaller (by height) addressbar.

1000 word image link :
https://i.postimg.cc/HW3PYJtW/ffox-tab-8px.png