Fix firefox addon_success needle

I updated this last month without looking closely enough; one of
the matches was bogus. We now don't seem to get the 'added to
Firefox' popup any more, so we can't match on it, but I guess we
can match on the 'remove' button as an indicator that the add
worked.

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

needles/firefox/addon_success-20200116.json

@@ -1,18 +1,18 @@
 {
     "area": [
         {
-      "ypos": 705,
+            "height": 15,
-      "width": 32,
             "type": "match",
-      "height": 34,
+            "width": 15,
-      "xpos": 58
+            "xpos": 504,
+            "ypos": 500
         },
         {
-      "width": 20,
-      "ypos": 71,
-      "xpos": 953,
             "height": 19,
-      "type": "match"
+            "type": "match",
+            "width": 20,
+            "xpos": 953,
+            "ypos": 71
         }
     ],
     "properties": [],