What are they meant to do?

The layout of Google Images results seems to fluctuate, so if it looks different to you at the moment, it could be an experiment. You could try clearing cache and Google cookies to see whether you get back to the previous iteration that the extensions know how to handle.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that.

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try reloading the page. Does that help?

jscher2000 said

What are they meant to do?

With a mouse hover, It magnifies an image in popup form and good quality.

I have another app, Zoom Image to Fit, that does it from the context menu, but the quality is poor.

I think this is a google issue.

Maybe someone knows of one an addon works?