Try Thumbnail Zoom Plus Rated 4 out of 5 stars
I've made a "forked" version of this very useful plug-in, which updates it to work again with Facebook and adds other enhancements. You can find it at https://addons.mozilla.org/en-US/firefox/addon/thumbnail-zoom-plus/ .
New forked version fixes problems Rated 3 out of 5 stars
Since the original Thumbnail Zoom hasn't been updated for a while, I made a forked version called Thumbnail Zoom Plus, which fixes some problems (including Facebook and Firefox 7), adds some sites including Reddit, and allows bigger thumbnails. You can install it from https://addons.mozilla.org/en-US/firefox/addon/thumbnail-zoom-plus .
Developers interested in examining the code can find it at https://github.com/dadler/thumbnail-zoom/