1. Open Web Inspector 2. Select an element in the Elements tab. 3. Right-click and select Capture Screenshot from the menu.
If you select <html> in the Elements tab you will capture a full-screen screenshot. Sometimes there are bugs, but it generally works.
https://derailer.org/paparazzi/ does that, unfortunately, macOS only.
Firefox also has a built-in mechanism to take scrolling-screenshots.
https://cleanshot.com/ is also very good, IMO, much better than Shottr.
SingleFile [1] is an open source browser extension that saves the webpage as a single offline HTML page. I personally prefer this to screenshots because you can do things like select text in the saved HTML page. Also if you don't trust the chrome web store or firefox addon repo, you can download the extension directly from Github.