If you're like me, and you hate the new Facebook Photo Viewer, this quick trick is for you. The new Facebook Photo Viewer is basically a prettified popup: it's slow, it covers up all the rest of your Facebook content, and you have to close it every time you want to get back to browsing the social network.
To get the old Facebook Photo Viewer back, all you have to do is reload the page. Once the new Facebook Photo Viewer has loaded, hit enter on the URL again, hit your browser's refresh button near the address bar, or just hit F5. Voilà!
From this point on, you can browse the rest of the album you already have open in the old Facebook Photo Viewer. Unfortunately, this solution is not permanent because every URL that points to a photo on Facebook now ends with "&theater" - this is what tells the service to use the new Facebook Photo Viewer.
Thankfully, Firefox or Chrome users can have this extra part stripped out of the URL. If you have the Greasemonkey add-on for Firefox or the Tampermonkey extension for Chrome, all you need to do is get the Facebook Photo Theater Killer userscript. Alternatively, there's also a Chrome extension that does the same thing: Revert Facebook Photo Viewer, which is what I personally use. Now all photos will automatically open up in the old Facebook Photo Viewer.
If you know any other plug-ins that get the old Facebook Photo Viewer back, do not hesitate to contact me or leave a comment below. The same goes for if you want me to post another Facebook workaround in general.