This repository contains some files to make Firefox customization less painful.
Required readings:
http://kb.mozillazine.org/User.js_file
The most popular way of setting advanced preferences. The file in this repository is a very watered down version of pyllyukko's user.js (for my convenience). You should definitely check out the original.
Required readings:
https://developer.mozilla.org/en-US/Firefox/Enterprise_deployment
Namely the mozilla.cfg and autoconfig.js files in the repository. You must know what every single preference you put in there does. Otherwise you're in for a lot of unwanted surprises. Best practice would be to get used to using user.js first and then attempt to set global preferences.
Required readings:
http://kb.mozillazine.org/index.php?title=UserChrome.css
http://kb.mozillazine.org/UserContent.css
It is used to apply styles to the browser itself. Mostly used to enforce themes.
https://en.wikipedia.org/wiki/Userscript
This repository also contains a few userscripts that can be used to apply styles to websites.
To use them, you will need a script manager first. I recommend using Greasemonkey or Violentmonkey. Regardless of what of you use, make sure that it's open-source.