In some cases, the background thumbnail service can produce thumbnails that it knows are bad, and Mark suggested the possibility of showing a placeholder or favicon then, assuming no thumbnail already exists (bug 901294 comment 35). Those cases are: the server response is not a 2xx, and the content was retargeted and not actually shown in the browser (e.g., a binary file). Bug 906615 will cause the bg service to never overwrite existing thumbnails, so this may be a smaller issue than we previously thought, but it's still possible if the foreground service declines to capture.