I need to blur the frame buffer and I don\'t know how to get the frame buffer using THREE.js.
I want to blur the whole frame buffer rather than blur each texture
Try using the MeshDepthMaterial and render this into your shader.
I suggest rendering the blur pass with a dedicated camera using the same settings as the scene's diffuse camera. Then by adjusting the camera's frustrum you can do both screen and depth of blur effects. For a screen setup move the near frustrum towards the camera and move the far frustrum in increments away from the camera.
http://threejs.org/docs/#Reference/Materials/MeshDepthMaterial