diff --git a/docshell/base/nsIContentViewer.idl b/docshell/base/nsIContentViewer.idl
index d7d58ad69cc1..8541454f3039 100644
--- a/docshell/base/nsIContentViewer.idl
+++ b/docshell/base/nsIContentViewer.idl
@@ -200,6 +200,9 @@ interface nsIContentViewer : nsISupports
   /** The amount by which to scale all text. Default is 1.0. */
   attribute float textZoom;
 
+  /** The actual text zoom in effect, as modified by the system font scale. */
+  readonly attribute float effectiveTextZoom;
+
   /** The amount by which to scale all lengths. Default is 1.0. */
   attribute float fullZoom;
 
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 2abdeaa14a35..dd72d3b6e406 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -3016,6 +3016,15 @@ nsDocumentViewer::GetTextZoom(float* aTextZoom)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDocumentViewer::GetEffectiveTextZoom(float* aEffectiveTextZoom)
+{
+  NS_ENSURE_ARG_POINTER(aEffectiveTextZoom);
+  nsPresContext* pc = GetPresContext();
+  *aEffectiveTextZoom = pc ? pc->EffectiveTextZoom() : 1.0f;
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDocumentViewer::SetMinFontSize(int32_t aMinFontSize)
 {
diff --git a/toolkit/content/widgets/browser.xml b/toolkit/content/widgets/browser.xml
index 3043e88022a7..4f0388ace5a9 100644
--- a/toolkit/content/widgets/browser.xml
+++ b/toolkit/content/widgets/browser.xml
@@ -552,6 +552,13 @@
         ]]></setter>
       </property>
 
+      <property name="effectiveTextZoom"
+                readonly="true">
+        <getter><![CDATA[
+          return this.markupDocumentViewer.effectiveTextZoom;
+        ]]></getter>
+      </property>
+
       <property name="isSyntheticDocument">
         <getter><![CDATA[
           return this.contentDocument.mozSyntheticDocument;