Add a couple more variant Firefox needles

needles/firefox/addon_install-20170816.json

@@ -0,0 +1,22 @@
+{
+  "properties": [],
+  "tags": [
+    "firefox_addon_install"
+  ],
+  "area": [
+    {
+      "height": 35,
+      "ypos": 143,
+      "xpos": 61,
+      "type": "match",
+      "width": 35
+    },
+    {
+      "height": 14,
+      "width": 26,
+      "xpos": 396,
+      "ypos": 277,
+      "type": "match"
+    }
+  ]
+}

needles/firefox/download_complete-20170816.json

@@ -0,0 +1,16 @@
+{
+  "tags": [
+    "browser_download_complete",
+    "firefox_download_complete"
+  ],
+  "properties": [],
+  "area": [
+    {
+      "height": 19,
+      "width": 34,
+      "type": "match",
+      "ypos": 103,
+      "xpos": 872
+    }
+  ]
+}