Rated 4 out of 5 stars

"Pop-up takes keyboard focus" - a truer statement has never been made, whether the box is checked or not.This is highly annoying for me, since my mouse is not on a level surface (can't be helped) and tends to drift. If it drifts over an image, whatever I was typing is lost and odds are Firefox is now opening a new tab. Annoying.However, that is my ONLY complaint about this add-on.

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Edit: Reply to suggestion
That seems to have done the trick. Thank you!
I was not using hotkeys, so it was no loss and all gain! :)

This review is for a previous version of the add-on (2.5.1-signed). 

Yes, focus can be a problem, especially on Linux. It may help to turn off the Thumbnail Zoom Plus "hotkeys" preference so you don't accidentally open images in new tabs (though you'll lose the benefit of hotkeys too).