It does work with FF51 if you follow Alex's advice! Rated 5 out of 5 stars

Well, over the last few days I too was frustrated that FireShot had stopped working with FF51. However, I followed Alex's advice that he posted today (1/25/17) and downloaded the file he pointed to . . . and my Firefox Pro is fully back!

This review is for a previous version of the add-on (0.98.89).