After using "Image Zoom" for quite some time, I finally found an alternative that works with Firefox 57+! Really hope options for downscaling (75% and 50%) would be added somewhere in the future :-)
Thank you TurboLion for your feedback. I have added downscaling to my backlog. I am not sure about the need of this use case, but it should be perfectly doable by enabling custom transformation in the add-on preferences, which should be perfect for this.
EDIT: Image downscaling has now been added in 1.4.
