fairlanguage-firefox/content_scripts/fairlang.js

// ---------------------------------
// Content script to be run on pages
// ---------------------------------

(function() {
	// Only run this script once
	if (window.hasRun) {
		return;
	}
	window.hasRun = true;

	// Search for all text areas
	document.querySelectorAll("textarea").forEach(function(textarea) {
		// Do something with it...
		console.log(textarea);
		textarea.style.border = "2px solid red";
	});

})();