Firefox only supports a full page screenshot that includes what you currently have loaded on the page. You can't include the next page or content loaded automatically if you scroll down the page in the same screenshot, only what is present when you take the screenshot is included.

• https://support.mozilla.org/en-US/kb/take-screenshots-firefox

I am not using the "full page screenshot"; I am dragging the screenshot's handle to highlight a portion of what's displayed on the screen. I want to use the horizontal and/or vertical scroll bars to include more content in the screenshot.

You should be able to drag the corner or right border to extend the selection to include more if Firefox doesn't scroll automatically. You can also drag the scroll bar to make more visible and adjust the selection as long are you see the border. Doesn't that work for you ?

neither works.

If you cannot scroll the page while a Screenshot selection is active -- using either the scroll bar or the mouse scroll wheel -- this can indicate that the page style rules lock the height of the body to the viewpoint height and add scrolling to an interior region of the page. The Screenshot feature does not work well with this design (full page screenshots do not work either). I don't know whether any add-ons can work around this, or whether it's necessary to "hack" the page design to capture the content.