diff --git a/Casks/firefox-nightly.rb b/Casks/firefox-nightly.rb
index bbcc037cb..0e3efa62a 100644
--- a/Casks/firefox-nightly.rb
+++ b/Casks/firefox-nightly.rb
@@ -1,5 +1,5 @@
 cask :v1 => 'firefox-nightly' do
-  version '38.0a1'
+  version '39.0a1'
   sha256 :no_check # required as upstream package is updated in-place
 
   url "https://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/firefox-#{version}.en-US.mac.dmg"