--- a/widget/gtk/nsLookAndFeel.cpp
+++ b/widget/gtk/nsLookAndFeel.cpp
@@ -736,17 +736,17 @@ GetSystemFontInfo(GtkStyleContext *aStyl
     if (!pango_font_description_get_size_is_absolute(desc)) {
         // |size| is in pango-points, so convert to pixels.
         size *= float(gfxPlatformGtk::GetFontScaleDPI()) / POINTS_PER_INCH_FLOAT;
     }
 
     // Scale fonts up on HiDPI displays.
     // This would be done automatically with cairo, but we manually manage
     // the display scale for platform consistency.
-    size *= ScreenHelperGTK::GetGTKMonitorScaleFactor();
+    size *= mozilla::widget::ScreenHelperGTK::GetGTKMonitorScaleFactor();
 
     // |size| is now pixels
 
     aFontStyle->size = size;
 
     pango_font_description_free(desc);
 }