diff --git a/pkgs/applications/networking/browsers/firefox/21.0.nix b/pkgs/applications/networking/browsers/firefox/20.0.nix
similarity index 97%
rename from pkgs/applications/networking/browsers/firefox/21.0.nix
rename to pkgs/applications/networking/browsers/firefox/20.0.nix
index 1a23c9d9cf9..1480a300820 100644
--- a/pkgs/applications/networking/browsers/firefox/21.0.nix
+++ b/pkgs/applications/networking/browsers/firefox/20.0.nix
@@ -15,9 +15,9 @@ assert stdenv.gcc ? libc && stdenv.gcc.libc != null;
 
 rec {
 
-  firefoxVersion = "21.0";
+  firefoxVersion = "20.0";
 
-  xulVersion = "21.0"; # this attribute is used by other packages
+  xulVersion = "20.0"; # this attribute is used by other packages
 
 
   src = fetchurl {
@@ -27,7 +27,7 @@ rec {
         # Fall back to this url for versions not available at releases.mozilla.org.
         "ftp://ftp.mozilla.org/pub/mozilla.org/firefox/releases/${firefoxVersion}/source/firefox-${firefoxVersion}.source.tar.bz2"
     ];
-    sha256 = "076rfh82m1b1hw10dzbgg13h63wmnispgyvag4nb20ylnlf0629r";
+    sha1 = "6d776c29da0be0d2a50abeb504d63b06b7861218";
   };
 
   commonConfigureFlags =
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 8493daf2de0..f87abe9082c 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -7192,7 +7192,7 @@ let
 
   firefoxWrapper = wrapFirefox { browser = pkgs.firefox; };
 
-  firefoxPkgs = pkgs.firefox21Pkgs;
+  firefoxPkgs = pkgs.firefox20Pkgs;
 
   firefox36Pkgs = callPackage ../applications/networking/browsers/firefox/3.6.nix {
     inherit (gnome) libIDL;
@@ -7213,12 +7213,12 @@ let
 
   firefox19Wrapper = lowPrio (wrapFirefox { browser = firefox19Pkgs.firefox; });
 
-  firefox21Pkgs = callPackage ../applications/networking/browsers/firefox/21.0.nix {
+  firefox20Pkgs = callPackage ../applications/networking/browsers/firefox/20.0.nix {
     inherit (gnome) libIDL;
     inherit (pythonPackages) pysqlite;
   };
 
-  firefox21Wrapper = lowPrio (wrapFirefox { browser = firefox21Pkgs.firefox; });
+  firefox20Wrapper = lowPrio (wrapFirefox { browser = firefox20Pkgs.firefox; });
 
   flac = callPackage ../applications/audio/flac { };
 
diff --git a/pkgs/top-level/release.nix b/pkgs/top-level/release.nix
index 32494f64ad2..11102f89155 100644
--- a/pkgs/top-level/release.nix
+++ b/pkgs/top-level/release.nix
@@ -359,7 +359,7 @@ let
       };
 
       firefox36Pkgs.firefox = linux;
-      firefox21Pkgs.firefox = linux;
+      firefox20Pkgs.firefox = linux;
 
       gnome = {
         gnome_panel = linux;