Aren't screenshots already built in? Ctrl-Shift-S or the scissor icon. Are you asking something different? (I *love* Firefox's current screenshot setup, fwiw.)
We are currently looking at options for making this kind of thing easier and safer. I don't expect we'll build in an image editor into Firefox, but it might be possible to better empower web extensions to provide this functionality while preserving a streamlined and intuitive flow that starts with taking a screenshot.
If you have specific examples of how you expect this would work, or examples of the kinds of uses you would put this to, please leave them here.