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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/