Fittr Flickr - Light Box
Screen shot of the new Fittr Flickr Light Box. After installing the extension you can click the image or press 'L' to get a large preview of the photo without changing pages.
Note that it won't work for non-public photos due to API permissions (I'll work on a fix) or images that don't have a Large or Original version available. Look for the magnify icon in the bottom-right when you hover over a photo.