Rated 4 out of 5 stars

- The ability to disable per site with version 3.1 cured a big pain. But you still can't exclude a site from the tool bar menu button; you have to open the extensions page, find this extension, open the options, find the corresponding section and type the name of the address of the site for each website! It is so cumbersome!
- Still the keyboard loses focus if you forget your mouse on a small image.
- If the thumbnail is the original image itself, and it is not loaded fully, TZP shows only the loaded part of the image (TZP assumes that the browser cache is always trustworty and uses the erroneous image data from there.).
- Has a very high miss rate; especially for large images.

Example:

Goto this URL: http://electronics.stackexchange.com/q/141739/5542

There are three images:
A) http://i.stack.imgur.com/FPxoi.jpg
1.228px × 694px (scaled to 630px × 356px)
B) http://i.stack.imgur.com/2347i.jpg
362px × 234px
C) http://i.stack.imgur.com/3lDxp.jpg
1.293px × 415px (scaled to 630px × 202px)

TZP opens images (2) and (3), but ignores the image (1).

This review is for a previous version of the add-on (3.2.1-signed). 

Rated 3 out of 5 stars

- Has a very high failure rate.
- If the original image is too large to fit on the screen, TZP doesn't show it.
- If the thumbnail is actually the original image itself, and it is not loaded fully, TZP shows only the loaded part of the image.
- VERY IMPORTANT!: Need option for per site enable/disable.
- VERY FRUSTRATING: If you leave the mouse pointer on an image before typing text in a text area, you can't type the text, because the text area loses the focus.
- VERY FRUSTRATING: Select a text and CTRL+C won't work if you forget the mouse pointer on an image.

This review is for a previous version of the add-on (2.7.1-signed). 

I'm sorry you're having trouble.
- If hotkeys are frustrating you, turn them off in Thumbnail Zoom Plus preferences.
- Version 3.1 now has an option to disable per-site.
- If an image is too large to fit, TZP does show it, but limits size to your screen size. If it's not showing an image at all, something else is wrong.