Alert Firefox users if they didn't set up flexbox correctly.

src/features.coffee

@@ -212,6 +212,16 @@ Settings =
       open: -> Conf['Enable 4chan\'s Extension']
 
     $.get 'previousversion', null, (item) ->
+      <% if (type === 'userscript') { %>
+      el = $.el 'span'
+      el.style.flex = 'test'
+      if el.style.flex is 'test'
+        el.innerHTML = """
+        Firefox is not correctly set up and some <%= meta.name %> features will be displayed incorrectly.<br>
+        Follow the instructions of the <a href='<%= meta.page %>' target=_blank>install guide</a> to fix it.
+        """
+        new Notification 'warning', el, 30
+      <% } %>
       if previous = item['previousversion']
         return if previous is g.VERSION
         # Avoid conflicts between sync'd newer versions