mkFirefoxModule: fix userChrome with leading comment (#6836)

userChrome lines that started with a leading comment would cause an eval failure.

modules/programs/firefox/mkFirefoxModule.nix

@@ -874,7 +874,11 @@ in
                 "chrome"
               else
                 "chrome/userChrome.css";
-            sourcePath = if lib.types.path.check profile.userChrome then profile.userChrome else null;
+            sourcePath =
+              if ((i: lib.isPath i && lib.types.path.check i) profile.userChrome) then
+                profile.userChrome
+              else
+                null;
           in
           # Merge the regular profile settings with extension settings
           mkMerge (

tests/modules/programs/firefox/profiles/userchrome/chrome/userChrome.css

@@ -1,3 +1,7 @@
+/*
+  This is a simple comment that should be written inside the `chrome/userChrome.css`
+*/
+
 #urlbar {
   min-width: none !important;
   border: none !important;

tests/modules/programs/firefox/profiles/userchrome/default.nix

@@ -18,14 +18,17 @@ in
         basic.isDefault = true;
         lines = {
           id = 1;
-          userChrome = # CSS
+          userChrome = ''
-            ''
+            /*
-              #urlbar {
+              This is a simple comment that should be written inside the `chrome/userChrome.css`
-                min-width: none !important;
+            */
-                border: none !important;
+
-                outline: none !important;
+            #urlbar {
-              }
+              min-width: none !important;
-            '';
+              border: none !important;
+              outline: none !important;
+            }
+          '';
         };
         path = {
           id = 2;

tests/modules/programs/firefox/profiles/userchrome/expected-userchrome.css

@@ -1,3 +1,7 @@
+/*
+  This is a simple comment that should be written inside the `chrome/userChrome.css`
+*/
+
 #urlbar {
   min-width: none !important;
   border: none !important;