diff --git a/pkgs/applications/networking/browsers/firefox-bin/default.nix b/pkgs/applications/networking/browsers/firefox-bin/default.nix
index c8f28b551cd7..b0c77819c077 100644
--- a/pkgs/applications/networking/browsers/firefox-bin/default.nix
+++ b/pkgs/applications/networking/browsers/firefox-bin/default.nix
@@ -50,7 +50,7 @@
 , ffmpeg
 , runtimeShell
 , mesa # firefox wants gbm for drm+dmabuf
-, systemLocale ? config.i18n.defaultLocale or "en-US"
+, systemLocale ? config.i18n.defaultLocale or "en_US"
 }:
 
 let
@@ -78,7 +78,12 @@ let
 
   defaultSource = lib.findFirst (sourceMatches "en-US") {} sources;
 
-  source = lib.findFirst (sourceMatches systemLocale) defaultSource sources;
+  mozLocale =
+    if systemLocale == "ca_ES@valencia"
+    then "ca-valencia"
+    else lib.replaceStrings ["_"] ["-"] systemLocale;
+
+  source = lib.findFirst (sourceMatches mozLocale) defaultSource sources;
 
   pname = "firefox-${channel}-bin-unwrapped";