Thanks to the Light Switch add-on for Firefox, you can hide any other content on the website you are looking for that is not some sort of multimedia. Once you install and start using this extension, you can quickly "turn off the lights" when watching videos or checking out pictures too.
How does the switching work? You get a light switch placed in the top-right corner of the multimedia in question. For example, if you were watching a video on YouTube, you’d see a little light switch towards the top-right side of the video player.
If you need a good distraction free way of viewing content, Light Switch is well worth checking out. You can download Light Switch from the Firefox Add-ons website.