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?
One use case that I understand is having the opportunity of saving source url as reference with the image. Jpg can save url in 'comments' box, png can't. Just mentioning this for casual information exchange. Not challenging the reality that 'JPEG is lossy'.
We are looking at including metadata in the image in Bug 1798543 - Screenshot Attribution and Annotation. The plan is to add an option to include the URL. That should work for PNG, however different applications surface different subsets of the available metadata for images, so we don't have much control over that side of things.