I'm curious why you would prefer a .jpg vs. a .png for a screenshot? The PNG format/compression tends to be a better fit for most web page content. JPEG is lossy and does a poor job of representing crisp edges needed for text and lines on a page. There's some history and context on the decision we made to remove JPEG as an option in bug 1649915.
What is the use-case? Is the screenshot image needed for some application that only accepts JPEG? Or is the page subject composed of photographic-style imagery better handled by JPEG?