diff --git a/mozilla/gfx/src/gtk/nsDrawingSurfaceGTK.cpp b/mozilla/gfx/src/gtk/nsDrawingSurfaceGTK.cpp
index a6d89e2e434..0bf1b2be91b 100644
--- a/mozilla/gfx/src/gtk/nsDrawingSurfaceGTK.cpp
+++ b/mozilla/gfx/src/gtk/nsDrawingSurfaceGTK.cpp
@@ -174,7 +174,10 @@ NS_IMETHODIMP nsDrawingSurfaceGTK :: Lock(PRInt32 aX, PRInt32 aY,
 
   *aBits = GDK_IMAGE_XIMAGE(mImage)->data;
 
-  *aWidthBytes = aWidth*(mImage->bpp >> 3);
+  // Use GDK_IMAGE_XIMAGE(mImage)->bits_per_pixel instead of mImage->bpp
+  // since, although bpp is documented as *bytes* per pixel, GDK1 sometimes
+  // set it as *bits* per pixel.
+  *aWidthBytes = aWidth * ((GDK_IMAGE_XIMAGE(mImage)->bits_per_pixel + 7) / 8);
   *aStride = GDK_IMAGE_XIMAGE(mImage)->bytes_per_line;
 
 #ifdef CHEAP_PERFORMANCE_MEASUREMENT