diff --git a/gfx/thebes/gfxUserFontSet.cpp b/gfx/thebes/gfxUserFontSet.cpp
index a1acf253188..97f940817b2 100644
--- a/gfx/thebes/gfxUserFontSet.cpp
+++ b/gfx/thebes/gfxUserFontSet.cpp
@@ -784,6 +784,7 @@ gfxUserFontEntry::GetUserFontSets(nsTArray<gfxUserFontSet*>& aResult)
 gfxUserFontSet::gfxUserFontSet()
     : mFontFamilies(4),
       mLocalRulesUsed(false),
+      mRebuildLocalRules(false),
       mDownloadCount(0),
       mDownloadSize(0)
 {
@@ -958,6 +959,7 @@ void
 gfxUserFontSet::RebuildLocalRules()
 {
     if (mLocalRulesUsed) {
+        mRebuildLocalRules = true;
         DoRebuildUserFontSet();
     }
 }
diff --git a/gfx/thebes/gfxUserFontSet.h b/gfx/thebes/gfxUserFontSet.h
index eb4720a5f36..88caa6ff442 100644
--- a/gfx/thebes/gfxUserFontSet.h
+++ b/gfx/thebes/gfxUserFontSet.h
@@ -526,6 +526,9 @@ protected:
     // true when local names have been looked up, false otherwise
     bool mLocalRulesUsed;
 
+    // true when rules using local names need to be redone
+    bool mRebuildLocalRules;
+
     // performance stats
     uint32_t mDownloadCount;
     uint64_t mDownloadSize;
diff --git a/layout/style/FontFaceSet.cpp b/layout/style/FontFaceSet.cpp
index 6a18c330dd6..9d07a8ce277 100644
--- a/layout/style/FontFaceSet.cpp
+++ b/layout/style/FontFaceSet.cpp
@@ -789,8 +789,11 @@ FontFaceSet::UpdateRules(const nsTArray<nsFontFaceRuleContainer>& aRules)
     CheckLoadingFinished();
   }
 
-  // local rules have been rebuilt, so clear the flag
-  mUserFontSet->mLocalRulesUsed = false;
+  // if local rules needed to be rebuilt, they have been rebuilt at this point
+  if (mUserFontSet->mRebuildLocalRules) {
+    mUserFontSet->mLocalRulesUsed = false;
+    mUserFontSet->mRebuildLocalRules = false;
+  }
 
   if (LOG_ENABLED() && !mRuleFaces.IsEmpty()) {
     LOG(("userfonts (%p) userfont rules update (%s) rule count: %d",
@@ -875,7 +878,8 @@ FontFaceSet::InsertRuleFontFace(FontFace* aFontFace, SheetType aSheetType,
 
       // if local rules were used, don't use the old font entry
       // for rules containing src local usage
-      if (mUserFontSet->mLocalRulesUsed) {
+      if (mUserFontSet->mLocalRulesUsed &&
+          mUserFontSet->mRebuildLocalRules) {
         nsCSSValue val;
         aFontFace->GetDesc(eCSSFontDesc_Src, val);
         nsCSSUnit unit = val.GetUnit();