Update compatibility details

package.json

@@ -3,7 +3,7 @@
   "author": "Thomas Rientjes",
   "license": "MPL-2.0",
   "title": "Decentraleyes",
-  "version": "1.3.8",
+  "version": "1.3.9",
   "main": "lib/main.js",
   "homepage": "https://addons.mozilla.org/firefox/addon/decentraleyes",
   "name": "decentraleyes",
@@ -13,8 +13,8 @@
     "multiprocess": true
   },
   "engines": {
-    "firefox": ">=38.0a1",
-    "fennec": ">=38.0a1",
+    "firefox": ">=51.0a1",
+    "fennec": ">=51.0a1",
     "seamonkey": ">=2.0a1 <=2.46",
     "{8de7fcbb-c55c-4fbe-bfc5-fc555c87dbc4}": ">=27.1.0b1"
   }