--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -174,16 +174,19 @@ typedef nsStyleTransformMatrix::Transfor
 
 /* static */ uint32_t nsLayoutUtils::sFontSizeInflationEmPerLine;
 /* static */ uint32_t nsLayoutUtils::sFontSizeInflationMinTwips;
 /* static */ uint32_t nsLayoutUtils::sFontSizeInflationLineThreshold;
 /* static */ int32_t  nsLayoutUtils::sFontSizeInflationMappingIntercept;
 /* static */ uint32_t nsLayoutUtils::sFontSizeInflationMaxRatio;
 /* static */ bool nsLayoutUtils::sFontSizeInflationForceEnabled;
 /* static */ bool nsLayoutUtils::sFontSizeInflationDisabledInMasterProcess;
+/* static */ uint32_t nsLayoutUtils::sSystemFontScale;
+/* static */ uint32_t nsLayoutUtils::sZoomMaxPercent;
+/* static */ uint32_t nsLayoutUtils::sZoomMinPercent;
 /* static */ bool nsLayoutUtils::sInvalidationDebuggingIsEnabled;
 /* static */ bool nsLayoutUtils::sCSSVariablesEnabled;
 /* static */ bool nsLayoutUtils::sInterruptibleReflowEnabled;
 /* static */ bool nsLayoutUtils::sSVGTransformBoxEnabled;
 /* static */ bool nsLayoutUtils::sTextCombineUprightDigitsEnabled;
 #ifdef MOZ_STYLO
 /* static */ bool nsLayoutUtils::sStyloEnabled;
 #endif
@@ -7711,16 +7714,22 @@ nsLayoutUtils::Initialize()
   Preferences::AddUintVarCache(&sFontSizeInflationLineThreshold,
                                "font.size.inflation.lineThreshold");
   Preferences::AddIntVarCache(&sFontSizeInflationMappingIntercept,
                               "font.size.inflation.mappingIntercept");
   Preferences::AddBoolVarCache(&sFontSizeInflationForceEnabled,
                                "font.size.inflation.forceEnabled");
   Preferences::AddBoolVarCache(&sFontSizeInflationDisabledInMasterProcess,
                                "font.size.inflation.disabledInMasterProcess");
+  Preferences::AddUintVarCache(&sSystemFontScale,
+                               "font.size.systemFontScale", 100);
+  Preferences::AddUintVarCache(&sZoomMaxPercent,
+                               "zoom.maxPercent", 300);
+  Preferences::AddUintVarCache(&sZoomMinPercent,
+                               "zoom.minPercent", 30);
   Preferences::AddBoolVarCache(&sInvalidationDebuggingIsEnabled,
                                "nglayout.debug.invalidation");
   Preferences::AddBoolVarCache(&sCSSVariablesEnabled,
                                "layout.css.variables.enabled");
   Preferences::AddBoolVarCache(&sInterruptibleReflowEnabled,
                                "layout.interruptible-reflow.enabled");
   Preferences::AddBoolVarCache(&sSVGTransformBoxEnabled,
                                "svg.transform-box.enabled");

--- a/layout/base/nsLayoutUtils.h
+++ b/layout/base/nsLayoutUtils.h
@@ -2428,16 +2428,32 @@ public:
   static bool FontSizeInflationForceEnabled() {
     return sFontSizeInflationForceEnabled;
   }
 
   static bool FontSizeInflationDisabledInMasterProcess() {
     return sFontSizeInflationDisabledInMasterProcess;
   }
 
+  /**
+   * See comment above "font.size.systemFontScale" in
+   * modules/libpref/init/all.js.
+   */
+  static float SystemFontScale() {
+    return sSystemFontScale / 100.0f;
+  }
+
+  static float MaxZoom() {
+    return sZoomMaxPercent / 100.0f;
+  }
+
+  static float MinZoom() {
+    return sZoomMinPercent / 100.0f;
+  }
+
   static bool SVGTransformBoxEnabled() {
     return sSVGTransformBoxEnabled;
   }
 
   static bool TextCombineUprightDigitsEnabled() {
     return sTextCombineUprightDigitsEnabled;
   }
 
@@ -2918,16 +2934,19 @@ public:
 private:
   static uint32_t sFontSizeInflationEmPerLine;
   static uint32_t sFontSizeInflationMinTwips;
   static uint32_t sFontSizeInflationLineThreshold;
   static int32_t  sFontSizeInflationMappingIntercept;
   static uint32_t sFontSizeInflationMaxRatio;
   static bool sFontSizeInflationForceEnabled;
   static bool sFontSizeInflationDisabledInMasterProcess;
+  static uint32_t sSystemFontScale;
+  static uint32_t sZoomMaxPercent;
+  static uint32_t sZoomMinPercent;
   static bool sInvalidationDebuggingIsEnabled;
   static bool sCSSVariablesEnabled;
   static bool sInterruptibleReflowEnabled;
   static bool sSVGTransformBoxEnabled;
   static bool sTextCombineUprightDigitsEnabled;
 #ifdef MOZ_STYLO
   static bool sStyloEnabled;
 #endif

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -3212,16 +3212,27 @@ pref("font.size.inflation.mappingInterce
  * i/s * 100 should never exceed the value of this preference.
  *
  * In order for this preference to have any effect, its value must be
  * greater than 100, since font inflation can never decrease the ratio
  * i/s.
  */
 pref("font.size.inflation.maxRatio", 0);
 
+/**
+ * This setting corresponds to a global text zoom setting affecting
+ * all content that is not already subject to font size inflation.
+ * It is interpreted as a percentage value that is applied on top
+ * of the document's current text zoom setting.
+ *
+ * The resulting total zoom factor (text zoom * system font scale)
+ * will be limited by zoom.minPercent and maxPercent.
+ */
+pref("font.size.systemFontScale", 100);
+
 /*
  * When enabled, the touch.radius and mouse.radius prefs allow events to be dispatched
  * to nearby elements that are sensitive to the event. See PositionedEventTargeting.cpp.
  * The 'mm' prefs define a rectangle around the nominal event target point within which
  * we will search for suitable elements. 'visitedWeight' is a percentage weight;
  * a value > 100 makes a visited link be treated as further away from the event target
  * than it really is, while a value < 100 makes a visited link be treated as closer
  * to the event target than it really is.