diff --git a/Casks/firefoxnightly.rb b/Casks/firefoxnightly.rb
index 324752b5f..e354eb137 100644
--- a/Casks/firefoxnightly.rb
+++ b/Casks/firefoxnightly.rb
@@ -1,5 +1,5 @@
 cask 'firefoxnightly' do
-  version '50.0a1'
+  version '51.0a1'
   sha256 :no_check # required as upstream package is updated in-place
 
   url "https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/firefox-#{version}.en-US.mac.dmg"