diff --git a/modules/programs/firefox/mkFirefoxModule.nix b/modules/programs/firefox/mkFirefoxModule.nix
index 1bb2253f1..cdbb96f7e 100644
--- a/modules/programs/firefox/mkFirefoxModule.nix
+++ b/modules/programs/firefox/mkFirefoxModule.nix
@@ -682,6 +682,12 @@ in {
           };
 
         };
+
+        config = let profile = config;
+        in {
+          settings."toolkit.legacyUserProfileCustomizations.stylesheets" =
+            mkIf (profile.userChrome != "" || profile.userContent != "") true;
+        };
       }));
       default = { };
       description = "Attribute set of ${name} profiles.";
@@ -739,7 +745,25 @@ in {
       (mkNoDuplicateAssertion cfg.profiles "profile")
     ] ++ (mapAttrsToList
       (_: profile: mkNoDuplicateAssertion profile.containers "container")
-      cfg.profiles);
+      cfg.profiles) ++ (
+        # Assert "toolkit.legacyUserProfileCustomizations.stylesheets" is enabled if
+        # userChrome/userContent is used.
+        let
+
+          assertProfile = userChromeAttr: profile:
+            profile.${userChromeAttr} != ""
+            -> (profile.settings."toolkit.legacyUserProfileCustomizations.stylesheets"
+              != false);
+
+          mkAssertion = userChromeAttr: {
+            assertion =
+              all (assertProfile userChromeAttr) (attrValues cfg.profiles);
+            message = ''
+              `${userChromeAttr}` won't work with `settings."toolkit.legacyUserProfileCustomizations.stylesheets"` set to false.
+            '';
+          };
+
+        in [ (mkAssertion "userChrome") (mkAssertion "userContent") ]);
 
     warnings = optional (cfg.enableGnomeExtensions or false) ''
       Using '${moduleName}.enableGnomeExtensions' has been deprecated and
diff --git a/tests/modules/programs/firefox/profile-settings.nix b/tests/modules/programs/firefox/profile-settings.nix
index 897067625..482ae95ed 100644
--- a/tests/modules/programs/firefox/profile-settings.nix
+++ b/tests/modules/programs/firefox/profile-settings.nix
@@ -15,6 +15,9 @@ let
       name = cfg.wrappedPackageName;
     };
 
+  userChromeExample = "#example-user-chrome { display: none; }";
+  userContentExample = "#example-user-content { display: none; }";
+
 in {
   imports = [ firefoxMockOverlay ];
 
@@ -160,12 +163,17 @@ in {
       id = 5;
       containers = {
         "shopping" = {
-          id = 6;
           icon = "circle";
           color = "yellow";
         };
       };
     };
+
+    profiles.userChrome = {
+      id = 6;
+      userChrome = userChromeExample;
+      userContent = userContentExample;
+    };
   } // {
 
     nmt.script = let
@@ -199,10 +207,6 @@ in {
         home-files/${cfg.configPath}/test/user.js \
         ${withName ./profile-settings-expected-user.js}
 
-      assertFileContent \
-        home-files/${cfg.configPath}/containers/containers.json \
-        ${withName ./profile-settings-expected-containers.json}
-
       bookmarksUserJs=$(normalizeStorePaths \
         home-files/${cfg.configPath}/bookmarks/user.js)
 
@@ -236,6 +240,22 @@ in {
       assertFirefoxSearchContent \
         home-files/${cfg.configPath}/searchWithoutDefault/search.json.mozlz4 \
         ${withName ./profile-settings-expected-search-without-default.json}
+
+      assertFileContent \
+        home-files/${cfg.configPath}/containers/containers.json \
+        ${withName ./profile-settings-expected-containers.json}
+
+      assertFileContains \
+        home-files/${cfg.configPath}/userChrome/user.js \
+        'user_pref("toolkit.legacyUserProfileCustomizations.stylesheets", true)'
+
+      assertFileContains \
+        home-files/${cfg.configPath}/userChrome/chrome/userChrome.css \
+        '${userChromeExample}'
+
+      assertFileContains \
+        home-files/${cfg.configPath}/userChrome/chrome/userContent.css \
+        '${userContentExample}'
     '';
   });
 }