Add new variant Firefox certificate error needle

It looks different in Fedora 30 update tests for some reason.

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

needles/firefox/firefox_certificate_error-20190308.json

@@ -0,0 +1,16 @@
+{
+  "area": [
+    {
+      "xpos": 799,
+      "height": 19,
+      "width": 75,
+      "type": "match",
+      "ypos": 508
+    }
+  ],
+  "properties": [],
+  "tags": [
+    "browser_certificate_error",
+    "firefox_certificate_error"
+  ]
+}