Firefox userChrome.css file that reduces the tab and navigation bar heights to increase available screen space. The thin bars have their buttons' and input fields' Tab selection highlight outline replaced by a browser accent color based background highlight, since the outline doesn't work well with the thinness.
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Visit about:config, search for toolkit.legacyUserProfileCustomizations.stylesheets and double-click to make it "true".
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Copy or append the contents of the userChrome.css file (in)to [your Firefox profile]/chrome/userChrome.css. (Open your profile directory by visiting about:profiles and clicking on the "Open Directory" button within the "Root Directory" row in the profile tagged "This is the profile in use and it cannot be deleted".)
Run the .sh file if on a Unix-like (Linux, Mac and others), or the .bat file (creation pending upon demand) if on Windows (usually by double-clicking) to have the manual install process performed automatically.
Given the simplicity in the install scripts the manual process is recommended if there are multiple Firefox installations in use.
The bar height is set by the --tab-max-height CSS variable at the top of the userChrome.css file.
