--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -229,16 +229,20 @@ pref("dom.script_loader.bytecode_cache.e
 pref("dom.script_loader.bytecode_cache.strategy", 0);
 
 // Fastback caching - if this pref is negative, then we calculate the number
 // of content viewers to cache based on the amount of available memory.
 pref("browser.sessionhistory.max_total_viewers", -1);
 
 pref("ui.use_native_colors", true);
 pref("ui.click_hold_context_menus", false);
+
+// Pop up context menu on mouseup instead of mousedown, if that's the OS default.
+// Note: ignored on Windows (context menus always use mouseup)
+pref("ui.context_menus.after_mouseup", false);
 // Duration of timeout of incremental search in menus (ms).  0 means infinite.
 pref("ui.menu.incremental_search.timeout", 1000);
 // If true, all popups won't hide automatically on blur
 pref("ui.popup.disable_autohide", false);
 
 pref("browser.display.use_document_fonts",  1);  // 0 = never, 1 = quick, 2 = always
 // 0 = default: always, except in high contrast mode
 // 1 = always

--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -4695,18 +4695,20 @@ NSEvent* gLastDragMouseDownEvent = nil;
   [self convertCocoaMouseEvent:theEvent toGeckoEvent:&geckoEvent];
   geckoEvent.button = WidgetMouseEvent::eRightButton;
   geckoEvent.mClickCount = [theEvent clickCount];
 
   mGeckoChild->DispatchInputEvent(&geckoEvent);
   if (!mGeckoChild)
     return;
 
-  // Let the superclass do the context menu stuff.
-  [super rightMouseDown:theEvent];
+  if (!nsBaseWidget::ShowContextMenuAfterMouseUp()) {
+    // Let the superclass do the context menu stuff.
+    [super rightMouseDown:theEvent];
+  }
 
   NS_OBJC_END_TRY_ABORT_BLOCK;
 }
 
 - (void)rightMouseUp:(NSEvent *)theEvent
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK;
 
@@ -4719,16 +4721,33 @@ NSEvent* gLastDragMouseDownEvent = nil;
   WidgetMouseEvent geckoEvent(true, eMouseUp, mGeckoChild,
                               WidgetMouseEvent::eReal);
   [self convertCocoaMouseEvent:theEvent toGeckoEvent:&geckoEvent];
   geckoEvent.button = WidgetMouseEvent::eRightButton;
   geckoEvent.mClickCount = [theEvent clickCount];
 
   nsAutoRetainCocoaObject kungFuDeathGrip(self);
   mGeckoChild->DispatchInputEvent(&geckoEvent);
+  if (!mGeckoChild)
+    return;
+
+  if (nsBaseWidget::ShowContextMenuAfterMouseUp()) {
+    // Let the superclass do the context menu stuff, but pretend it's rightMouseDown.
+    NSEvent *dupeEvent = [NSEvent mouseEventWithType:NSRightMouseDown
+                                            location:theEvent.locationInWindow
+                                       modifierFlags:theEvent.modifierFlags
+                                           timestamp:theEvent.timestamp
+                                        windowNumber:theEvent.windowNumber
+                                             context:theEvent.context
+                                         eventNumber:theEvent.eventNumber
+                                          clickCount:theEvent.clickCount
+                                            pressure:theEvent.pressure];
+
+    [super rightMouseDown:dupeEvent];
+  }
 
   NS_OBJC_END_TRY_ABORT_BLOCK;
 }
 
 - (void)rightMouseDragged:(NSEvent*)theEvent
 {
   if (!mGeckoChild)
     return;

--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -2733,16 +2733,29 @@ nsWindow::InitButtonEvent(WidgetMouseEve
 }
 
 static guint ButtonMaskFromGDKButton(guint button)
 {
     return GDK_BUTTON1_MASK << (button - 1);
 }
 
 void
+nsWindow::DispatchContextMenuEventFromMouseEvent(uint16_t domButton,
+                                                 GdkEventButton *aEvent)
+{
+    if (domButton == WidgetMouseEvent::eRightButton && MOZ_LIKELY(!mIsDestroyed)) {
+        WidgetMouseEvent contextMenuEvent(true, eContextMenu, this,
+                                          WidgetMouseEvent::eReal);
+        InitButtonEvent(contextMenuEvent, aEvent);
+        contextMenuEvent.pressure = mLastMotionPressure;
+        DispatchInputEvent(&contextMenuEvent);
+    }
+}
+
+void
 nsWindow::OnButtonPressEvent(GdkEventButton *aEvent)
 {
     LOG(("Button %u press on %p\n", aEvent->button, (void *)this));
 
     // If you double click in GDK, it will actually generate a second
     // GDK_BUTTON_PRESS before sending the GDK_2BUTTON_PRESS, and this is
     // different than the DOM spec.  GDK puts this in the queue
     // programatically, so it's safe to assume that if there's a
@@ -2801,23 +2814,18 @@ nsWindow::OnButtonPressEvent(GdkEventBut
     WidgetMouseEvent event(true, eMouseDown, this, WidgetMouseEvent::eReal);
     event.button = domButton;
     InitButtonEvent(event, aEvent);
     event.pressure = mLastMotionPressure;
 
     DispatchInputEvent(&event);
 
     // right menu click on linux should also pop up a context menu
-    if (domButton == WidgetMouseEvent::eRightButton &&
-        MOZ_LIKELY(!mIsDestroyed)) {
-        WidgetMouseEvent contextMenuEvent(true, eContextMenu, this,
-                                          WidgetMouseEvent::eReal);
-        InitButtonEvent(contextMenuEvent, aEvent);
-        contextMenuEvent.pressure = mLastMotionPressure;
-        DispatchInputEvent(&contextMenuEvent);
+    if (!nsBaseWidget::ShowContextMenuAfterMouseUp()) {
+        DispatchContextMenuEventFromMouseEvent(domButton, aEvent);
     }
 }
 
 void
 nsWindow::OnButtonReleaseEvent(GdkEventButton *aEvent)
 {
     LOG(("Button %u release on %p\n", aEvent->button, (void *)this));
 
@@ -2843,16 +2851,21 @@ nsWindow::OnButtonReleaseEvent(GdkEventB
     event.button = domButton;
     InitButtonEvent(event, aEvent);
     gdouble pressure = 0;
     gdk_event_get_axis ((GdkEvent*)aEvent, GDK_AXIS_PRESSURE, &pressure);
     event.pressure = pressure ? pressure : mLastMotionPressure;
 
     DispatchInputEvent(&event);
     mLastMotionPressure = pressure;
+
+    // right menu click on linux should also pop up a context menu
+    if (nsBaseWidget::ShowContextMenuAfterMouseUp()) {
+        DispatchContextMenuEventFromMouseEvent(domButton, aEvent);
+    }
 }
 
 void
 nsWindow::OnContainerFocusInEvent(GdkEventFocus *aEvent)
 {
     LOGFOCUS(("OnContainerFocusInEvent [%p]\n", (void *)this));
 
     // Unset the urgency hint, if possible

--- a/widget/gtk/nsWindow.h
+++ b/widget/gtk/nsWindow.h
@@ -240,16 +240,18 @@ private:
     LayoutDeviceIntSize GetSafeWindowSize(LayoutDeviceIntSize aSize);
 
     void               EnsureGrabs  (void);
     void               GrabPointer  (guint32 aTime);
     void               ReleaseGrabs (void);
 
     void               UpdateClientOffset();
 
+    void               DispatchContextMenuEventFromMouseEvent(uint16_t domButton,
+                                                              GdkEventButton *aEvent);
 public:
     void               ThemeChanged(void);
     void               OnDPIChanged(void);
     void               OnCheckResize(void);
     void               OnCompositedChanged(void);
 
 #ifdef MOZ_X11
     Window             mOldFocusWindow;

--- a/widget/nsBaseWidget.cpp
+++ b/widget/nsBaseWidget.cpp
@@ -1213,16 +1213,32 @@ nsBaseWidget::DispatchEventToAPZOnly(moz
   if (mAPZC) {
     MOZ_ASSERT(APZThreadUtils::IsControllerThread());
     uint64_t inputBlockId = 0;
     ScrollableLayerGuid guid;
     mAPZC->ReceiveInputEvent(*aEvent, &guid, &inputBlockId);
   }
 }
 
+// static
+bool
+nsBaseWidget::ShowContextMenuAfterMouseUp()
+{
+  static bool gContextMenuAfterMouseUp = false;
+  static bool gContextMenuAfterMouseUpCached = false;
+  if (!gContextMenuAfterMouseUpCached) {
+    Preferences::AddBoolVarCache(&gContextMenuAfterMouseUp,
+                                 "ui.context_menus.after_mouseup",
+                                 false);
+
+    gContextMenuAfterMouseUpCached = true;
+  }
+  return gContextMenuAfterMouseUp;
+}
+
 nsIDocument*
 nsBaseWidget::GetDocument() const
 {
   if (mWidgetListener) {
     if (nsIPresShell* presShell = mWidgetListener->GetPresShell()) {
       return presShell->GetDocument();
     }
   }

--- a/widget/nsBaseWidget.h
+++ b/widget/nsBaseWidget.h
@@ -412,16 +412,22 @@ public:
   void NotifyLiveResizeStopped();
 
 #if defined(MOZ_WIDGET_ANDROID)
   void RecvToolbarAnimatorMessageFromCompositor(int32_t) override {};
   void UpdateRootFrameMetrics(const ScreenPoint& aScrollOffset, const CSSToScreenScale& aZoom) override {};
   void RecvScreenPixels(mozilla::ipc::Shmem&& aMem, const ScreenIntSize& aSize) override {};
 #endif
 
+  /**
+   * Whether context menus should only appear on mouseup instead of mousedown,
+   * on OSes where they normally appear on mousedown (macOS, *nix).
+   */
+  static bool ShowContextMenuAfterMouseUp();
+
 protected:
   // These are methods for CompositorWidgetWrapper, and should only be
   // accessed from that class. Derived widgets can choose which methods to
   // implement, or none if supporting out-of-process compositing.
   virtual bool PreRender(mozilla::widget::WidgetRenderingContext* aContext) {
     return true;
   }
   virtual void PostRender(mozilla::widget::WidgetRenderingContext* aContext)