It is an extension of Google Chrome. In the same classic Facebook photo viewer that you are using can open photos. It works in a salient mode, you would get notification on the top-left corner while you installed this extension. Open any photo to check the extension is working or not. It has allowed you to browse Facebook album by using the old view mode.
This extension does not transmit, read or store any of your data. It modifies property of each photo links and runs on the background efficiently. It acts only on Facebook pages and has no fancy photo viewer button, icons or settings. The photo will appear on its own page and complete with like option and the full comment section.