diff --git a/layout/style/nsRuleNode.cpp b/layout/style/nsRuleNode.cpp
index dab8e883aea..de8540690dc 100644
--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -1556,6 +1556,16 @@ nsRuleNode::Transition(nsIStyleRule* aRule, uint8_t aLevel,
   return next;
 }
 
+nsRuleNode*
+nsRuleNode::RuleTree()
+{
+  nsRuleNode* n = this;
+  while (n->mParent) {
+    n = n->mParent;
+  }
+  return n;
+}
+
 void nsRuleNode::SetUsedDirectly()
 {
   mDependentBits |= NS_RULE_NODE_USED_DIRECTLY;
diff --git a/layout/style/nsRuleNode.h b/layout/style/nsRuleNode.h
index 3529e1be6ce..7ffa327f892 100644
--- a/layout/style/nsRuleNode.h
+++ b/layout/style/nsRuleNode.h
@@ -689,6 +689,12 @@ public:
   nsRuleNode* GetParent() const { return mParent; }
   bool IsRoot() const { return mParent == nullptr; }
 
+  // Return the root of the rule tree that this rule node is in.
+  nsRuleNode* RuleTree();
+  const nsRuleNode* RuleTree() const {
+    return const_cast<nsRuleNode*>(this)->RuleTree();
+  }
+
   // These uint8_ts are really nsStyleSet::sheetType values.
   uint8_t GetLevel() const {
     NS_ASSERTION(!IsRoot(), "can't call on root");
diff --git a/layout/style/nsStyleSet.cpp b/layout/style/nsStyleSet.cpp
index b3403162a01..05e34267430 100644
--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -233,10 +233,6 @@ nsStyleSet::EndReconstruct()
   mInReconstruct = false;
 #ifdef DEBUG
   for (int32_t i = mRoots.Length() - 1; i >= 0; --i) {
-    nsRuleNode *n = mRoots[i]->RuleNode();
-    while (n->GetParent()) {
-      n = n->GetParent();
-    }
     // Since nsStyleContext's mParent and mRuleNode are immutable, and
     // style contexts own their parents, and nsStyleContext asserts in
     // its constructor that the style context and its parent are in the
@@ -244,7 +240,8 @@ nsStyleSet::EndReconstruct()
     // mRoots; we only need to check the rule nodes of mRoots
     // themselves.
 
-    NS_ASSERTION(n == mRuleTree, "style context has old rule node");
+    NS_ASSERTION(mRoots[i]->RuleNode()->RuleTree() == mRuleTree,
+                 "style context has old rule node");
   }
 #endif
   // This *should* destroy the only element of mOldRuleTrees, but in