I suggest you talk to the sender about the size of the images they send you.

This to me sounds like getting a package from Ebay and complaining to the courier that the box is to large.

Photo attachments coming into Windows Live Mail are the right size; those coming in on Mozilla are too large. Why the difference and how can I reduce the size of the Mozilla photos?

I have no idea how windows live mail works... probably by scaling the image to fit the window, but I am only guessing.

Thunderbird displays attached images at the size they are sent. Perhaps you can find an add-on that scales images for you. https://addons.mozilla.org/en-US/thunderbird/.