diff --git a/content/canvas/src/WebGLContext.h b/content/canvas/src/WebGLContext.h
index d2b2d0d21f8b..1fa253668859 100644
--- a/content/canvas/src/WebGLContext.h
+++ b/content/canvas/src/WebGLContext.h
@@ -127,9 +127,10 @@ struct WebGLTexelPremultiplicationOp {
 
 int GetWebGLTexelFormat(GLenum format, GLenum type);
 
+// Zero is not an integer power of two.
 inline bool is_pot_assuming_nonnegative(WebGLsizei x)
 {
-    return (x & (x-1)) == 0;
+    return x && (x & (x-1)) == 0;
 }
 
 /* Each WebGL object class WebGLFoo wants to:
@@ -585,6 +586,7 @@ public:
     nsresult ErrorOutOfMemory(const char *fmt = 0, ...);
 
     const char *ErrorName(GLenum error);
+    bool IsTextureFormatCompressed(GLenum format);
 
     nsresult DummyFramebufferOperation(const char *info);
 
@@ -1247,14 +1249,26 @@ public:
 
     class ImageInfo : public WebGLRectangleObject {
     public:
-        ImageInfo() : mFormat(0), mType(0), mIsDefined(false) {}
+        ImageInfo()
+            : mFormat(0)
+            , mType(0)
+            , mIsDefined(false)
+        {}
+
         ImageInfo(WebGLsizei width, WebGLsizei height,
                   WebGLenum format, WebGLenum type)
-            : WebGLRectangleObject(width, height), mFormat(format), mType(type), mIsDefined(true) {}
+            : WebGLRectangleObject(width, height)
+            , mFormat(format)
+            , mType(type)
+            , mIsDefined(true)
+        {}
 
         bool operator==(const ImageInfo& a) const {
-            return mWidth == a.mWidth && mHeight == a.mHeight &&
-                   mFormat == a.mFormat && mType == a.mType;
+            return mIsDefined == a.mIsDefined &&
+                   mWidth     == a.mWidth &&
+                   mHeight    == a.mHeight &&
+                   mFormat    == a.mFormat &&
+                   mType      == a.mType;
         }
         bool operator!=(const ImageInfo& a) const {
             return !(*this == a);
diff --git a/content/canvas/src/WebGLContextGL.cpp b/content/canvas/src/WebGLContextGL.cpp
index 13365d8684d1..8532c7d83c30 100644
--- a/content/canvas/src/WebGLContextGL.cpp
+++ b/content/canvas/src/WebGLContextGL.cpp
@@ -1911,15 +1911,20 @@ WebGLContext::GenerateMipmap(WebGLenum target)
     WebGLTexture *tex = activeBoundTextureForTarget(target);
 
     if (!tex)
-        return ErrorInvalidOperation("generateMipmap: no texture is bound to this target");
+        return ErrorInvalidOperation("generateMipmap: No texture is bound to this target.");
 
-    if (!tex->IsFirstImagePowerOfTwo()) {
-        return ErrorInvalidOperation("generateMipmap: the width or height of this texture is not a power of two");
-    }
+    if (!tex->HasImageInfoAt(0, 0))
+        return ErrorInvalidOperation("generateMipmap: Level zero of texture is not defined.");
 
-    if (!tex->AreAllLevel0ImageInfosEqual()) {
-        return ErrorInvalidOperation("generateMipmap: the six faces of this cube map have different dimensions, format, or type.");
-    }
+    if (!tex->IsFirstImagePowerOfTwo())
+        return ErrorInvalidOperation("generateMipmap: Level zero of texture does not have power-of-two width and height.");
+
+    GLenum format = tex->ImageInfoAt(0, 0).Format();
+    if (IsTextureFormatCompressed(format))
+        return ErrorInvalidOperation("generateMipmap: Texture data at level zero is compressed.");
+
+    if (!tex->AreAllLevel0ImageInfosEqual())
+        return ErrorInvalidOperation("generateMipmap: The six faces of this cube map have different dimensions, format, or type.");
 
     tex->SetGeneratedMipmap();
 
diff --git a/content/canvas/src/WebGLContextUtils.cpp b/content/canvas/src/WebGLContextUtils.cpp
index 898fb9a05fea..f448fe138eb7 100644
--- a/content/canvas/src/WebGLContextUtils.cpp
+++ b/content/canvas/src/WebGLContextUtils.cpp
@@ -239,4 +239,27 @@ WebGLContext::ErrorName(GLenum error)
             NS_ABORT();
             return "[unknown WebGL error!]";
     }
-};
+}
+
+bool
+WebGLContext::IsTextureFormatCompressed(GLenum format)
+{
+    switch(format) {
+        case LOCAL_GL_RGB:
+        case LOCAL_GL_RGBA:
+        case LOCAL_GL_ALPHA:
+        case LOCAL_GL_LUMINANCE:
+        case LOCAL_GL_LUMINANCE_ALPHA:
+            return false;
+
+        case LOCAL_GL_COMPRESSED_RGB_S3TC_DXT1_EXT:
+        case LOCAL_GL_COMPRESSED_RGBA_S3TC_DXT1_EXT:
+        case LOCAL_GL_COMPRESSED_RGBA_S3TC_DXT3_EXT:
+        case LOCAL_GL_COMPRESSED_RGBA_S3TC_DXT5_EXT:
+            return true;
+    }
+
+    NS_NOTREACHED("Invalid WebGL texture format?");
+    NS_ABORT();
+    return false;
+}