Sometimes people send you so large images, that when you click on "View Image" that you see only a small portion of the picture. You first of all need to save it and open it in an image viewer to be able to see it properly. Mozilla has all the functionality to resize images in it, there is just no way to say "Display Image so it fits in window". I guess this is a dupe, but after half an hour of Mozilla searching, I found nothing.
I think this is the one that you wanted: bug 73322 "Automatic image resizing" *** This bug has been marked as a duplicate of 73322 ***
I looked at the other bug, and it is indeed a dupe of this. => Verifying dupe