--- a/content/canvas/src/WebGLContextValidate.cpp
+++ b/content/canvas/src/WebGLContextValidate.cpp
@@ -876,16 +876,28 @@ bool WebGLContext::ValidateStencilParams
   }
   if (mStencilWriteMaskFront != mStencilWriteMaskBack) {
       ErrorInvalidOperation(msg, "stencilMaskSeparate", "write masks");
       return false;
   }
   return true;
 }
 
+static inline int32_t floorPOT(int32_t x)
+{
+    MOZ_ASSERT(x > 0);
+    int32_t pot = 1;
+    while (pot < 0x40000000) {
+        if (x < pot*2)
+            break;
+        pot *= 2;
+    }
+    return pot;
+}
+
 bool
 WebGLContext::InitAndValidateGL()
 {
     if (!gl) return false;
 
     GLenum error = gl->fGetError();
     if (error != LOCAL_GL_NO_ERROR) {
         GenerateWarning("GL error 0x%x occurred during OpenGL context initialization, before WebGL initialization!", error);
@@ -955,16 +967,19 @@ WebGLContext::InitAndValidateGL()
     } else {
         gl->fGetIntegerv(LOCAL_GL_MAX_TEXTURE_SIZE, &mGLMaxTextureSize);
         gl->fGetIntegerv(LOCAL_GL_MAX_CUBE_MAP_TEXTURE_SIZE, &mGLMaxCubeMapTextureSize);
         gl->fGetIntegerv(LOCAL_GL_MAX_RENDERBUFFER_SIZE, &mGLMaxRenderbufferSize);
         gl->fGetIntegerv(LOCAL_GL_MAX_TEXTURE_IMAGE_UNITS, &mGLMaxTextureImageUnits);
         gl->fGetIntegerv(LOCAL_GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS, &mGLMaxVertexTextureImageUnits);
     }
 
+    mGLMaxTextureSize = floorPOT(mGLMaxTextureSize);
+    mGLMaxRenderbufferSize = floorPOT(mGLMaxRenderbufferSize);
+
     if (MinCapabilityMode()) {
         mGLMaxFragmentUniformVectors = MINVALUE_GL_MAX_FRAGMENT_UNIFORM_VECTORS;
         mGLMaxVertexUniformVectors = MINVALUE_GL_MAX_VERTEX_UNIFORM_VECTORS;
         mGLMaxVaryingVectors = MINVALUE_GL_MAX_VARYING_VECTORS;
     } else {
         if (gl->HasES2Compatibility()) {
             gl->fGetIntegerv(LOCAL_GL_MAX_FRAGMENT_UNIFORM_VECTORS, &mGLMaxFragmentUniformVectors);
             gl->fGetIntegerv(LOCAL_GL_MAX_VERTEX_UNIFORM_VECTORS, &mGLMaxVertexUniformVectors);