Add code for simple Firefox extension

README.md

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+# Firefox: First Extension Tutorial (Borderify)
+Following Mozilla's [Your first extension](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Your_first_WebExtension).
+
+## Install
+Navigate to ```about:debugging```, select This Firefox, Load Temporary Add-on, and select any file in this directory.
+
+Now navigate to any mozilla.org page, like https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Your_first_WebExtension

borderify.js

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+document.body.style.border = "5px solid red";

manifest.json

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+{
+	"manifest_version": 2,
+	"name": "Borderify",
+	"version": "1.0",
+
+	"description": "Adds a red border to all webpages matching mozilla.org.",
+
+	"icons": {
+	  "48": "icons/border-48.png"
+	},
+
+	"content_scripts": [
+	  {
+		"matches": ["*://*.mozilla.org/*"],
+		"js": ["borderify.js"]
+	  }
+	]
+}