Add pinned firefox version to the general settings in test_matrix.json

Change-Id: I376126ed28a2c90e3a164f0e6c36b9e2004575ef
Reviewed-on: https://dart-review.googlesource.com/62809
Reviewed-by: Alexander Thomas <athom@google.com>
diff --git a/tools/bots/test_matrix.json b/tools/bots/test_matrix.json
index df0b50c..f3bdae3 100644
--- a/tools/bots/test_matrix.json
+++ b/tools/bots/test_matrix.json
@@ -1,6 +1,7 @@
 {
   "global": {
-    "chrome": "65"
+    "chrome": "65",
+    "ff": "61"
   },
   "filesets": {
     "analyzer": [