Add firefox config setup script

.config/firefox/setup

@@ -0,0 +1,3 @@
+for i in ~/.mozilla/firefox/*.*default*
+  do ln -s user.js "$i/user.js"
+done

.config/firefox/user.js

@@ -2,9 +2,13 @@
  * Note: has to be symlinked to profile directories for your
  *       firefox release
  *
+ * E.g.
+ *
  * ```sh
- * ln user.js ~/.mozilla/firefox/*.default-release/user.js
+ * ln -s user.js ~/.mozilla/firefox/*.default-release/user.js
  * ```
+ * Or check out the `setup` script
+ *
  */
 
 user_pref('browser.download.dir', '/home/h/dl')