Currently, if a person tries to save a web page, they have a couple choices built-in to Firefox:
There is also the option in most Windows computers to "Print to PDF" in some way, but this usually doesn't print the entire page, or it doesn't print the current view.
Firefox should give users the choice to save an image (in a standard image format) or a PDF of the current view, including any portion of the web page which is available by scrolling but isn't visible on the screen.
It is often important to capture details from a current view that the save and screenshot features simply cannot capture. The only way to accomplish this now is by laboriously screenshotting and scrolling and screenshotting and scrolling until all of the page is in images, then stitching those images together in some image manipulation software.
It should not be difficult to create the ability to take a screenshot of the entire page in image or PDF formats. If Firefox can simulate many different screen sizes in "Responsive Design Mode", there is no particular reason why it must do so on the screen itself, but this could be done in memory alone for the purpose of saving the page.
Please add a true screenshot feature to Firefox!