Try using an old version of Imagus like 0.9.8.52, last workable version for my FF 52.
Imagus asks server for larger images by default, while TZP only zooms the thumbnail in the cache. Totally different "thumbnail"s.
Just in case some are too lazy to tweak around, enable advanced options, set "Activation with right mouse button" to "click", and "Minimum size-difference to zoom" to 0%, just so you can right click to put the popup at the center, then scroll up and down to zoom, or, when working with images that are in full sizes already, use Ctrl+Right Click to force a popup, then right click on that again. Use this whenever it won't show popups.
By default you can press 3 or 4 to take up full width or height of your browser to display images, NOT your entire screen like TZP does.