diff --git a/components/style/properties/data.py b/components/style/properties/data.py
index 0b2debb233d..0597662d520 100644
--- a/components/style/properties/data.py
+++ b/components/style/properties/data.py
@@ -70,7 +70,7 @@ class Keyword(object):
         moz_stripped = value.replace("-moz-", '') if self.gecko_strip_moz_prefix else value.replace("-moz-", 'moz-')
         mapped = self.consts_map.get(value)
         if self.gecko_enum_prefix:
-            parts = moz_stripped.split('-')
+            parts = moz_stripped.replace('-', '_').split('_')
             parts = mapped if mapped else [p.title() for p in parts]
             return self.gecko_enum_prefix + "::" + "".join(parts)
         else:
diff --git a/components/style/properties/longhand/ui.mako.rs b/components/style/properties/longhand/ui.mako.rs
index 79d25467df6..8b8d902c37d 100644
--- a/components/style/properties/longhand/ui.mako.rs
+++ b/components/style/properties/longhand/ui.mako.rs
@@ -16,11 +16,12 @@ ${helpers.single_keyword("ime-mode", "normal auto active disabled inactive",
                          animatable=False,
                          spec="https://drafts.csswg.org/css-ui/#input-method-editor")}
 
-${helpers.single_keyword("-moz-user-select", "auto text none all", products="gecko",
+${helpers.single_keyword("-moz-user-select", "auto text none all element elements" +
+                            " toggle tri_state -moz-all -moz-none -moz-text",
+                         products="gecko",
                          alias="-webkit-user-select",
                          gecko_ffi_name="mUserSelect",
                          gecko_enum_prefix="StyleUserSelect",
-                         gecko_inexhaustive=True,
                          animatable=False,
                          spec="https://drafts.csswg.org/css-ui-4/#propdef-user-select")}
 
diff --git a/tests/unit/style/parsing/mod.rs b/tests/unit/style/parsing/mod.rs
index f4bd0fad644..cfaabc6c3c4 100644
--- a/tests/unit/style/parsing/mod.rs
+++ b/tests/unit/style/parsing/mod.rs
@@ -94,3 +94,4 @@ mod selectors;
 mod supports;
 mod text_overflow;
 mod transition_timing_function;
+mod ui;
diff --git a/tests/unit/style/parsing/ui.rs b/tests/unit/style/parsing/ui.rs
new file mode 100644
index 00000000000..c364e47bce5
--- /dev/null
+++ b/tests/unit/style/parsing/ui.rs
@@ -0,0 +1,32 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use cssparser::Parser;
+use media_queries::CSSErrorReporterTest;
+use servo_url::ServoUrl;
+use style::parser::ParserContext;
+use style::stylesheets::Origin;
+use style_traits::ToCss;
+
+#[test]
+fn test_moz_user_select() {
+    use style::properties::longhands::_moz_user_select;
+
+    assert_roundtrip_with_context!(_moz_user_select::parse, "auto");
+    assert_roundtrip_with_context!(_moz_user_select::parse, "text");
+    assert_roundtrip_with_context!(_moz_user_select::parse, "none");
+    assert_roundtrip_with_context!(_moz_user_select::parse, "element");
+    assert_roundtrip_with_context!(_moz_user_select::parse, "elements");
+    assert_roundtrip_with_context!(_moz_user_select::parse, "toggle");
+    assert_roundtrip_with_context!(_moz_user_select::parse, "tri_state");
+    assert_roundtrip_with_context!(_moz_user_select::parse, "-moz-all");
+    assert_roundtrip_with_context!(_moz_user_select::parse, "-moz-none");
+    assert_roundtrip_with_context!(_moz_user_select::parse, "-moz-text");
+
+    let url = ServoUrl::parse("http://localhost").unwrap();
+    let context = ParserContext::new(Origin::Author, &url, Box::new(CSSErrorReporterTest));
+
+    let mut negative = Parser::new("potato");
+    assert!(_moz_user_select::parse(&context, &mut negative).is_err());
+}