diff --git a/gfx/thebes/gfxPlatform.cpp b/gfx/thebes/gfxPlatform.cpp
index 2f9b866ec5d6..12ff7d487638 100644
--- a/gfx/thebes/gfxPlatform.cpp
+++ b/gfx/thebes/gfxPlatform.cpp
@@ -566,6 +566,9 @@ static void WebRenderDebugPrefChangeCallback(const char* aPrefName, void*) {
   GFX_WEBRENDER_DEBUG(".picture-caching", 1 << 15)
   GFX_WEBRENDER_DEBUG(".texture-cache.disable-shrink", 1 << 16)
   GFX_WEBRENDER_DEBUG(".primitives", 1 << 17)
+  // Bit 18 is for the zoom display, which requires the mouse position and thus
+  // currently only works in wrench.
+  GFX_WEBRENDER_DEBUG(".small-screen", 1 << 19)
 #undef GFX_WEBRENDER_DEBUG
 
   gfx::gfxVars::SetWebRenderDebugFlags(flags);
diff --git a/gfx/wr/webrender/src/debug_render.rs b/gfx/wr/webrender/src/debug_render.rs
index 5b28c3593c31..0ab47363d41e 100644
--- a/gfx/wr/webrender/src/debug_render.rs
+++ b/gfx/wr/webrender/src/debug_render.rs
@@ -313,6 +313,7 @@ impl DebugRenderer {
         &mut self,
         device: &mut Device,
         viewport_size: Option<FramebufferIntSize>,
+        scale: f32,
     ) {
         if let Some(viewport_size) = viewport_size {
             device.disable_depth();
@@ -321,8 +322,8 @@ impl DebugRenderer {
 
             let projection = Transform3D::ortho(
                 0.0,
-                viewport_size.width as f32,
-                viewport_size.height as f32,
+                viewport_size.width as f32 * scale,
+                viewport_size.height as f32 * scale,
                 0.0,
                 ORTHO_NEAR_PLANE,
                 ORTHO_FAR_PLANE,
diff --git a/gfx/wr/webrender/src/renderer.rs b/gfx/wr/webrender/src/renderer.rs
index 4649883e77d6..6d7f579a3aa3 100644
--- a/gfx/wr/webrender/src/renderer.rs
+++ b/gfx/wr/webrender/src/renderer.rs
@@ -2782,7 +2782,9 @@ impl Renderer {
             let _gm = self.gpu_profile.start_marker("end frame");
             self.gpu_profile.end_frame();
             if let Some(debug_renderer) = self.debug.try_get_mut() {
-                debug_renderer.render(&mut self.device, framebuffer_size);
+                let small_screen = self.debug_flags.contains(DebugFlags::SMALL_SCREEN);
+                let scale = if small_screen { 1.6 } else { 1.0 };
+                debug_renderer.render(&mut self.device, framebuffer_size, scale);
             }
             // See comment for texture_resolver.begin_frame() for explanation
             // of why this must be done after all rendering, including debug
diff --git a/gfx/wr/webrender_api/src/api.rs b/gfx/wr/webrender_api/src/api.rs
index 61f0220f73e0..ac592b26f5c0 100644
--- a/gfx/wr/webrender_api/src/api.rs
+++ b/gfx/wr/webrender_api/src/api.rs
@@ -1040,6 +1040,10 @@ bitflags! {
         const PRIMITIVE_DBG = 1 << 17;
         /// Draw a zoom widget showing part of the framebuffer zoomed in.
         const ZOOM_DBG = 1 << 18;
+        /// Scale the debug renderer down for a smaller screen. This will disrupt
+        /// any mapping between debug display items and page content, so shouldn't
+        /// be used with overlays like the picture caching or primitive display.
+        const SMALL_SCREEN = 1 << 19;
     }
 }
 
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 15e556187e1e..8b054a21ba6e 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -965,6 +965,7 @@ pref("gfx.webrender.debug.show-overdraw", false);
 pref("gfx.webrender.debug.slow-frame-indicator", false);
 pref("gfx.webrender.debug.picture-caching", false);
 pref("gfx.webrender.debug.primitives", false);
+pref("gfx.webrender.debug.small-screen", false);
 pref("gfx.webrender.dl.dump-parent", false);
 pref("gfx.webrender.dl.dump-content", false);
 pref("gfx.webrender.picture-caching", true);