As others have said, with the Firefox update from 51.0 to 51.0.1, the add-on is currently not working. Opening an instagram photo or video in its tab (as usual) no longer shows the save button that was previously available. I assume this will be addressed as soon as lumerias can reproduce. :)
Like the others, I am on 51.0.1, although I am using the 64-bit flavor.
Edit: Downloaded the updated version (1.4) and this appears to be fixed. Updating the rating accordingly!