You don’t even need to open Responsive Design Mode - when you select Take Screenshot, there are two buttons “Save visible” and “Save full page” in the top right-hand corner.
Legit one of the most underrated Firefox features that I use all the time: right-click -> Take Screenshot (or Ctrl+Shift+S). No need to look up the relevant node, just hover the relevant part with your cursor.
Y’all, there’s a full video, this single image doesn’t do it justice: https://birdbutt.com/@barnibu/110995778137311940
Ah yes, RDM is a clever workaround for that - I should remember that.