diff --git a/components/style/dom.rs b/components/style/dom.rs
index 4c57edcb1c5..ed8b52f02ef 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -19,7 +19,6 @@ use media_queries::Device;
 use properties::{AnimationRules, ComputedValues, PropertyDeclarationBlock};
 #[cfg(feature = "gecko")] use properties::LonghandId;
 #[cfg(feature = "gecko")] use properties::animated_properties::AnimationValue;
-#[cfg(feature = "gecko")] use properties::animated_properties::TransitionProperty;
 use rule_tree::CascadeLevel;
 use selector_parser::{AttrValue, ElementExt};
 use selector_parser::{PseudoClassStringArg, PseudoElement};
@@ -651,7 +650,7 @@ pub trait TElement : Eq + PartialEq + Debug + Hash + Sized + Copy + Clone +
     /// Gets the current existing CSS transitions, by |property, end value| pairs in a FnvHashMap.
     #[cfg(feature = "gecko")]
     fn get_css_transitions_info(&self)
-                                -> FnvHashMap<TransitionProperty, Arc<AnimationValue>>;
+                                -> FnvHashMap<LonghandId, Arc<AnimationValue>>;
 
     /// Does a rough (and cheap) check for whether or not transitions might need to be updated that
     /// will quickly return false for the common case of no transitions specified or running. If
@@ -684,7 +683,7 @@ pub trait TElement : Eq + PartialEq + Debug + Hash + Sized + Copy + Clone +
         combined_duration: f32,
         before_change_style: &ComputedValues,
         after_change_style: &ComputedValues,
-        existing_transitions: &FnvHashMap<TransitionProperty, Arc<AnimationValue>>
+        existing_transitions: &FnvHashMap<LonghandId, Arc<AnimationValue>>
     ) -> bool;
 
     /// Returns the value of the `xml:lang=""` attribute (or, if appropriate,
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 7d82c6dddbc..6e1d875548a 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -1303,19 +1303,30 @@ impl<'le> TElement for GeckoElement<'le> {
 
     fn get_css_transitions_info(
         &self,
-    ) -> FnvHashMap<TransitionProperty, Arc<AnimationValue>> {
+    ) -> FnvHashMap<LonghandId, Arc<AnimationValue>> {
         use gecko_bindings::bindings::Gecko_ElementTransitions_EndValueAt;
         use gecko_bindings::bindings::Gecko_ElementTransitions_Length;
         use gecko_bindings::bindings::Gecko_ElementTransitions_PropertyAt;
 
         let collection_length =
-            unsafe { Gecko_ElementTransitions_Length(self.0) };
-        let mut map = FnvHashMap::with_capacity_and_hasher(collection_length, Default::default());
+            unsafe { Gecko_ElementTransitions_Length(self.0) } as usize;
+        let mut map = FnvHashMap::with_capacity_and_hasher(
+            collection_length,
+            Default::default()
+        );
+
         for i in 0..collection_length {
-            let (property, raw_end_value) = unsafe {
-                (Gecko_ElementTransitions_PropertyAt(self.0, i as usize).into(),
-                 Gecko_ElementTransitions_EndValueAt(self.0, i as usize))
+            let property = unsafe {
+                Gecko_ElementTransitions_PropertyAt(self.0, i as usize)
             };
+
+            let property = LonghandId::from_nscsspropertyid(property)
+                .expect("Only longhands should be in the element transitions");
+
+            let raw_end_value = unsafe {
+                 Gecko_ElementTransitions_EndValueAt(self.0, i)
+            };
+
             let end_value = AnimationValue::arc_from_borrowed(&raw_end_value);
             debug_assert!(end_value.is_some());
             map.insert(property, end_value.unwrap().clone_arc());
@@ -1363,7 +1374,7 @@ impl<'le> TElement for GeckoElement<'le> {
         after_change_style: &ComputedValues
     ) -> bool {
         use gecko_bindings::structs::nsCSSPropertyID;
-        use std::collections::HashSet;
+        use properties::LonghandIdSet;
 
         debug_assert!(self.might_need_transitions_update(Some(before_change_style),
                                                          after_change_style),
@@ -1381,13 +1392,7 @@ impl<'le> TElement for GeckoElement<'le> {
                    property == nsCSSPropertyID::eCSSProperty_UNKNOWN;
         };
 
-        let mut transitions_to_keep = if !existing_transitions.is_empty() &&
-                                         (after_change_box_style.transition_nscsspropertyid_at(0) !=
-                                              nsCSSPropertyID::eCSSPropertyExtra_all_properties) {
-            Some(HashSet::<TransitionProperty>::with_capacity(transitions_count))
-        } else {
-            None
-        };
+        let mut transitions_to_keep = LonghandIdSet::new();
 
         for i in 0..transitions_count {
             let property = after_change_box_style.transition_nscsspropertyid_at(i);
@@ -1400,7 +1405,8 @@ impl<'le> TElement for GeckoElement<'le> {
 
             let transition_property: TransitionProperty = property.into();
 
-            let property_check_helper = |property: &LonghandId| -> bool {
+            let mut property_check_helper = |property: &LonghandId| -> bool {
+                transitions_to_keep.insert(*property);
                 self.needs_transitions_update_per_property(
                     property,
                     combined_duration,
@@ -1427,17 +1433,13 @@ impl<'le> TElement for GeckoElement<'le> {
                         return true;
                     }
                 },
-            };
-
-            if let Some(ref mut transitions_to_keep) = transitions_to_keep {
-                transitions_to_keep.insert(transition_property);
             }
         }
 
         // Check if we have to cancel the running transition because this is not
         // a matching transition-property value.
-        transitions_to_keep.map_or(false, |set| {
-            existing_transitions.keys().any(|property| !set.contains(property))
+        existing_transitions.keys().any(|property| {
+            !transitions_to_keep.contains(*property)
         })
     }
 
@@ -1447,7 +1449,7 @@ impl<'le> TElement for GeckoElement<'le> {
         combined_duration: f32,
         before_change_style: &ComputedValues,
         after_change_style: &ComputedValues,
-        existing_transitions: &FnvHashMap<TransitionProperty, Arc<AnimationValue>>,
+        existing_transitions: &FnvHashMap<LonghandId, Arc<AnimationValue>>,
     ) -> bool {
         use values::animated::{Animate, Procedure};
 
@@ -1457,13 +1459,10 @@ impl<'le> TElement for GeckoElement<'le> {
         // If the end value has not changed, we should leave the currently
         // running transition as-is since we don't want to interrupt its timing
         // function.
-        //
-        // FIXME(emilio): The shorthand / longhand mapping with transitions
-        // looks pretty fishy!
-        if let Some(ref existing) = existing_transitions.get(&TransitionProperty::Longhand(*longhand_id)) {
+        if let Some(ref existing) = existing_transitions.get(longhand_id) {
             let after_value =
                 AnimationValue::from_computed_values(
-                    &longhand_id,
+                    longhand_id,
                     after_change_style
                 ).unwrap();