Update Firefox addon_add needle, icon on button disappeared

I think this is in upstream page source, not on our end.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

needles/firefox/addon_add-20210513.json

@@ -0,0 +1,15 @@
+{
+  "area": [
+    {
+      "height": 18,
+      "width": 33,
+      "ypos": 523,
+      "type": "match",
+      "xpos": 475
+    }
+  ],
+  "properties": [],
+  "tags": [
+    "firefox_addon_add"
+  ]
+}