As I said somewhere else, to get more compact tabs you can go to about:config and search for a setting called browser.tabs.tabMinWidth, I usually change the number to 20 (the default minimum width is like 70) and tabs are allowed to become roughly as narrow as in chrome. And if by "more compact tab bar" you meant how tall tabs are, there's the browser.compactmode.show setting, put it to "true" and then in the Firefox menu under More Tools → Customize Toolbars you can select "compact mode" in the "Density" menu on the bottom, which makes the tab bar and toolbars shorter