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It does not seem to work. Try with this picture: http://www.nikonclub.it/forum/index.php?act=attach&type=post&id=95247

ExifViewer, another firefox add-on (that I DO NOT like), is able to show exif data on this image.

Try also this one: http://www.nikonclub.it/forum/index.php?act=attach&type=post&id=95243

Questa recensione riguarda una versione precedente (0.2.3) del componente aggiuntivo. 

I was able to reproduce this. If the image doesn't fit into the cache (Options/Page Info/Media/Size says "Unknown (not cached)") it didn't work.
Version 0.3 can now scan the file directly over the network (if connected). Depending on your connection and where in the file the EXIF data is (at the begin or at the end) this may take some time but it's the only way I know of.