awesome! Rated 5 out of 5 stars

Eliot, thank you a lot for changing the trigger button to RMB.
However, RMB still opens the context menu upon releasing the button.
I also found that this consequence doesn't trigger context menu: RMB down, LMB down, LMB up, RMB up.
(where down = push and hold the button, and up = release it).

Since you didn't give up development (phew!), here is another bug report:
After zooming a picture in/out, sometimes it is needed to set it's initial size. In 'Image Zoom' extension this combo served for good: click MMB while holding RMB.
In your extension - it just sends a middle click to the image, so if the image is an URL - you'll get a new tab opened with that URL. The same for LMB (while RMB is being held).
I think you could use "hit MMB while RMB is held" to resize the image to it's initial state. And if it's already in it's initial state and the user again hits MMB while RMB is held - you could maximize the image to fit the screen.

Thank you for improving your extension, the latest version now replaced me the old, outdated, buggy, leaky, non-restartless Image Zoom.

This review is for a previous version of the add-on (1.2.rev653.1-signed.1-signed).  This user has other reviews of this add-on.

I can't reproduce those issues

This is weird, for me the RMB doesn't open the context menu unless I did not move the mouse.

You can restore images by just clicking with RMB if the image was resized before.

Not sure what you mean with the MMB, are you talking about maximizing the image? That's not a recommended feature, I just put it there for exceptional cases.

[Edit]: You can try out v1.3 from my website