diff --git a/modules/programs/firefox/mkFirefoxModule.nix b/modules/programs/firefox/mkFirefoxModule.nix
index f22af019b..8475c95a6 100644
--- a/modules/programs/firefox/mkFirefoxModule.nix
+++ b/modules/programs/firefox/mkFirefoxModule.nix
@@ -102,7 +102,8 @@ let
       ${builtins.toJSON {
         version = 4;
         lastUserContextId =
-          elemAt (mapAttrsToList (_: container: container.id) containers) 0;
+          foldlAttrs (acc: _: value: if value.id > acc then value.id else acc) 0
+          containers;
         identities = mapAttrsToList containerToIdentity containers ++ [
           {
             userContextId = 4294967294; # 2^32 - 2