firefox-userChrome

UPDATE
From Firefox 69 on you will need to set a flag in about:config. Otherwise the userChrome will be disabled.
Set toolkit.legacyUserProfileCustomizations.stylesheetsto true.
However: It seems that moz-binding was disabled as well, which means you can't style the scrollbars anymore... :(

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This is my current configuration of the Firefox userChrome (located in your profile folder – yes, you have this, too).
What's awesome here is it even styles the scrollbars! See paragraph above. This is not longer in use for me.

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Have a look this gist of mine, where I collect some of the hidden flags in about:config you should be aware of.