diff --git a/components/style/gecko/pseudo_element.rs b/components/style/gecko/pseudo_element.rs
index a08da8d40f1..73fe905ebd8 100644
--- a/components/style/gecko/pseudo_element.rs
+++ b/components/style/gecko/pseudo_element.rs
@@ -14,6 +14,7 @@ use properties::{ComputedValues, PropertyFlags};
 use properties::longhands::display::computed_value::T as Display;
 use selector_parser::{NonTSPseudoClass, PseudoElementCascadeType, SelectorImpl};
 use std::fmt;
+use str::starts_with_ignore_ascii_case;
 use string_cache::Atom;
 use thin_slice::ThinBoxedSlice;
 use values::serialize_atom_identifier;
diff --git a/components/style/gecko/pseudo_element_definition.mako.rs b/components/style/gecko/pseudo_element_definition.mako.rs
index be0318eadea..dd27dfd5676 100644
--- a/components/style/gecko/pseudo_element_definition.mako.rs
+++ b/components/style/gecko/pseudo_element_definition.mako.rs
@@ -242,9 +242,7 @@ impl PseudoElement {
                 return Some(PseudoElement::Placeholder);
             }
             _ => {
-                // FIXME: -moz-tree check should probably be
-                // ascii-case-insensitive.
-                if name.starts_with("-moz-tree-") {
+                if starts_with_ignore_ascii_case(name, "-moz-tree-") {
                     return PseudoElement::tree_pseudo_element(name, Box::new([]))
                 }
             }
diff --git a/components/style/gecko/selector_parser.rs b/components/style/gecko/selector_parser.rs
index fa514835e8c..cc84f58aa13 100644
--- a/components/style/gecko/selector_parser.rs
+++ b/components/style/gecko/selector_parser.rs
@@ -17,6 +17,7 @@ use selectors::parser::{SelectorParseErrorKind, Visit};
 use selectors::parser::{self as selector_parser, Selector};
 use selectors::visitor::SelectorVisitor;
 use std::fmt;
+use str::starts_with_ignore_ascii_case;
 use string_cache::{Atom, Namespace, WeakAtom, WeakNamespace};
 use style_traits::{CssWriter, ParseError, StyleParseErrorKind, ToCss as ToCss_};
 use thin_slice::ThinBoxedSlice;
@@ -468,8 +469,7 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         name: CowRcStr<'i>,
         parser: &mut Parser<'i, 't>,
     ) -> Result<PseudoElement, ParseError<'i>> {
-        // FIXME: -moz-tree check should probably be ascii-case-insensitive.
-        if name.starts_with("-moz-tree-") {
+        if starts_with_ignore_ascii_case(&name, "-moz-tree-") {
             // Tree pseudo-elements can have zero or more arguments, separated
             // by either comma or space.
             let mut args = Vec::new();