In a bugzilla entry asking for the incorporation of WebP, Mozilla developer Joe Drew said in a comment "As the WebP image format exists currently, I won't accept a patch for it". Readers were directed to a blog posting by Jeff Muizelaar for the rationale for the rejection. He explained that Mozilla is unhappy with the quality versus size claims made by Google for WebP and questioned the methodology used to test: this converted existing JPEG images into WebP images and then compared quality using PSNR (Peak Signal-to-Noise Ratio) and file size.
A month after Muizelaar made these comments, Google released a new study which used PNG images as source and compared quality using SSIM (Structural Similarity). This new study showed WebP images to be on average 25%-34% smaller than similar quality JPEGs, but the quality value is calculated and there is some discussion over whether WebP imagery is perceived by the untrained eye to be of the same quality.
Source: H online.