Everforest од natabot

For furher customization of newtab and caption buttons navigate to chrome folder (or create if it doesn't exist) inside your Firefox Profile folder and drop inside userChrome.css and userContent.css found here.

How to find chrome folder from Firefox browser:
Help > More troubleshooting information > Open folder (beside Profile folder field) > chrome (create if it doesn't exist)

!!! For Firefox to be able to load the chrome folder !!! :
+ Load about:config in the Firefox address bar (ignore the warning)
+ Then search for toolkit.legacyUserProfileCustomizations.stylesheets
+ Set it to True by double-clicking the "Value" column.

Restart Firefox for changes to take effect.