Rated 4 out of 5 stars

Excellent add-on. Would give it 5 stars except for the following bug:

Must remove or replace the "?" in the automatically generated file name with something else as Windows doesn't recognize the "?" as part of a valid file name. I replaced the "?" with a "+" and it saved just fine. Or, you can give it a new and proper file name.

Example:
"70663.png?modified=1339889479..png" was successfully saved as "70663.png+modified=1339889479..png"

NOTE: Just installed version 2.1 and it does the same thing.
Also, it seems the double period (..) is still there as seen in my example above, although it doesn't seem to present any problems with Windows.

This review is for a previous version of the add-on (2.0.1-signed). 

Fixed in 2.2beta1

Hello, this is fixed in version 2.2beta1. It now uses a default filename based on the caption text rather than the image's web address, and it removes any illegal characters from it. If it works for you, I hope you'll update your review. Thanks.

https://addons.mozilla.org/en-US/firefox/addon/thumbnail-zoom-plus/versions/?page=1#version-2.2beta1