I don't quite understand the 'screenshot' option because it offers two options (Save full page and Save visible) and in my opinion they do the same thing.
'Save visible' just saves what you can seen on your screen at that moment in time, whereas 'Save full page' will also save anything on the page that you can only see by scrolling the page - i.e. anything above or below the 'visible' portion of the page that you're currently viewing.
If I have an image in a tab and zoom in on that tab, the buttons are also affected by the zoom when using the screenshot option, making them difficult to see depending on the zoom level used.
@Josem : "save visible" is what you can currently see; "save full page" is everything, including what you cannot currently see, but could scroll to.
However... "save full page" cannot work on dynamic pages; those are ones where as you scroll-down, new stuff only loads-in at that time, and previous content now behind you gets unloaded. There is little Firefox could do to capture all of such pages.
Two workarounds if you use Windows:
1. Snipping Tool: in Windows 11, the Snipping Tool was updated to include a video-record option; it records for about 30 seconds. So you could start recording, then scroll the page down.
2. Problem Steps Recorder: in Windows 10, and some versions of Windows 11 (it's scheduled to get removed), it takes a screenshot every time you click. When you first start it, click the small down-arrow next to the help icon, then go to Settings, and increase the "Number of recent screen captures to store" to 99. Then click OK, then click "Start record". Click "Stop record" when done, and save. To find it, type psr or steps recorder when the Start Menu is open to search for it.