--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -10857,16 +10857,22 @@ PresShell::SetupFontInflation()
   NotifyFontSizeInflationEnabledIsDirty();
 }
 
 void
 nsIPresShell::RecomputeFontSizeInflationEnabled()
 {
   mFontSizeInflationEnabledIsDirty = false;
 
+  RecomputeDocEligibleForFontSizeInflation();
+  if (!mDocIsFontSizeInflationEligible) {
+    mFontSizeInflationEnabled = false;
+    return;
+  }
+
   MOZ_ASSERT(mPresContext, "our pres context should not be null");
   if ((FontSizeInflationEmPerLine() == 0 &&
       FontSizeInflationMinTwips() == 0) || mPresContext->IsChrome()) {
     mFontSizeInflationEnabled = false;
     return;
   }
 
   // Force-enabling font inflation always trumps the heuristics here.
@@ -10883,16 +10889,22 @@ nsIPresShell::RecomputeFontSizeInflation
       // explicitly disabled.
       if (FontSizeInflationDisabledInMasterProcess()) {
         mFontSizeInflationEnabled = false;
         return;
       }
     }
   }
 
+  mFontSizeInflationEnabled = true;
+}
+
+void
+nsIPresShell::RecomputeDocEligibleForFontSizeInflation()
+{
   // XXXjwir3:
   // See bug 706918, comment 23 for more information on this particular section
   // of the code. We're using "screen size" in place of the size of the content
   // area, because on mobile, these are close or equal. This will work for our
   // purposes (bug 706198), but it will need to be changed in the future to be
   // more correct when we bring the rest of the viewport code into platform.
   // We actually want the size of the content area, in the event that we don't
   // have any metadata about the width and/or height. On mobile, the screen size
@@ -10903,36 +10915,36 @@ nsIPresShell::RecomputeFontSizeInflation
 
   // TODO:
   // Once bug 716575 has been resolved, this code should be changed so that it
   // does the right thing on all platforms.
   nsresult rv;
   nsCOMPtr<nsIScreenManager> screenMgr =
     do_GetService("@mozilla.org/gfx/screenmanager;1", &rv);
   if (!NS_SUCCEEDED(rv)) {
-    mFontSizeInflationEnabled = false;
+    mDocIsFontSizeInflationEligible = false;
     return;
   }
 
   nsCOMPtr<nsIScreen> screen;
   screenMgr->GetPrimaryScreen(getter_AddRefs(screen));
   if (screen) {
     int32_t screenLeft, screenTop, screenWidth, screenHeight;
     screen->GetRect(&screenLeft, &screenTop, &screenWidth, &screenHeight);
 
     nsViewportInfo vInf =
       GetDocument()->GetViewportInfo(ScreenIntSize(screenWidth, screenHeight));
 
     if (vInf.GetDefaultZoom() >= CSSToScreenScale(1.0f) || vInf.IsAutoSizeEnabled()) {
-      mFontSizeInflationEnabled = false;
+      mDocIsFontSizeInflationEligible = false;
       return;
     }
   }
 
-  mFontSizeInflationEnabled = true;
+  mDocIsFontSizeInflationEligible = true;
 }
 
 bool
 nsIPresShell::FontSizeInflationEnabled()
 {
   if (mFontSizeInflationEnabledIsDirty) {
     RecomputeFontSizeInflationEnabled();
     PRES_LOG("%p: recomputed FontSizeInflationEnabled: %s\n", this, mFontSizeInflationEnabled?"true":"false");

--- a/layout/base/nsIPresShell.h
+++ b/layout/base/nsIPresShell.h
@@ -1628,16 +1628,23 @@ protected:
 
   /**
    * Do computations necessary to determine if font size inflation is enabled.
    * This value is cached after computation, as the computation is somewhat
    * expensive.
    */
   void RecomputeFontSizeInflationEnabled();
 
+  /**
+    * Determine if the underlying document is eligible for font size inflation.
+    * This value is cached after computation, as the computation is somewhat
+    * expensive.
+    */
+  void RecomputeDocEligibleForFontSizeInflation();
+
   void RecordAlloc(void* aPtr) {
 #ifdef DEBUG
     MOZ_ASSERT(!mAllocatedPointers.Contains(aPtr));
     mAllocatedPointers.PutEntry(aPtr);
 #endif
   }
 
   void RecordFree(void* aPtr) {
@@ -1822,16 +1829,17 @@ protected:
   // Cached font inflation values. This is done to prevent changing of font
   // inflation until a page is reloaded.
   uint32_t mFontSizeInflationEmPerLine;
   uint32_t mFontSizeInflationMinTwips;
   uint32_t mFontSizeInflationLineThreshold;
   bool mFontSizeInflationForceEnabled;
   bool mFontSizeInflationDisabledInMasterProcess;
   bool mFontSizeInflationEnabled;
+  bool mDocIsFontSizeInflationEligible;
   bool mPaintingIsFrozen;
 
   // Dirty bit indicating that mFontSizeInflationEnabled needs to be recomputed.
   bool mFontSizeInflationEnabledIsDirty;
 
   // If a document belongs to an invisible DocShell, this flag must be set
   // to true, so we can avoid any paint calls for widget related to this
   // presshell.