diff --git a/content/canvas/src/nsCanvasRenderingContext2D.cpp b/content/canvas/src/nsCanvasRenderingContext2D.cpp
index c8bdee8a5c2..7ece7e781cf 100644
--- a/content/canvas/src/nsCanvasRenderingContext2D.cpp
+++ b/content/canvas/src/nsCanvasRenderingContext2D.cpp
@@ -609,11 +609,14 @@ nsCanvasRenderingContext2D::StyleColorToString(const nscolor& aColor, nsAString&
                                         NS_GET_B(aColor)),
                         aStr);
     } else {
-        CopyUTF8toUTF16(nsPrintfCString(100, "rgba(%d,%d,%d,%0.2f)",
+        // "%0.5f" in nsPrintfCString would use the locale-specific
+        // decimal separator. That's why we have to do this:
+        PRUint32 alpha = NS_GET_A(aColor) * 100000 / 255;
+        CopyUTF8toUTF16(nsPrintfCString(100, "rgba(%d, %d, %d, 0.%d)",
                                         NS_GET_R(aColor),
                                         NS_GET_G(aColor),
                                         NS_GET_B(aColor),
-                                        NS_GET_A(aColor) / 255.0f),
+                                        alpha),
                         aStr);
     }
 }