This behavor - clicking on a zoomed in/out image resetting zoom to 100% - is default Firefox behavior for images (similar to how the image is drawn centered on the page and on a dark gray background). Using the built-in "open image in new tab" has the same behavior.

The addon does not have control the browser's rendering/behavior of the image, all it does it just open the image. It's unlikely that add-ons could alter the zoom behavior of the browser, at least not without some very hacky workarounds liable to break between firefox updates.

At this point, I suspect your best bet is to file a ticket on bugzilla, as this sounds like its a UX issue for touch-based pinch-zoom and panning of an image accidentally causing a click event to reset the zoom level. Sorry.