Whenever you fullscreen a video in Firefox, a warning message appears for a few seconds and then slides off the screen. While its primary purpose is for security, you might want to disable the popup if you find it annoying.

What Is the Fullscreen Warning, and Why Is It Important?

All modern browsers use the Fullscreen API to present desired content—such as videos, pictures, and online games—using the entire screen, void of other user interface elements. When enabled, a small warning message momentarily appears at the top of the page, notifying you it has been triggered. This is the same functionality when you watch a YouTube video in fullscreen mode.

While it can be annoying at times—and before you go ahead and disable the message altogether— it’s important to review its purpose and how it helps prevent phishing attacks.

The only real restriction for developers using the API is that in order to trigger fullscreen mode, the user must initiate it with a click or a keypress (keyboard shortcut). The reason this condition exists is to prevent sketchy sites from automatically loading a phishing scam into fullscreen when visiting a site.

Unfortunately, this feature is susceptible to a very ingenious phishing attack. Attackers can use the API to show you a fake version of a website in fullscreen mode that looks surprisingly similar to the real deal. All you need to do is click a legitimate-looking link, and the API loads fullscreen mode with what looks like a working web browser.

In this case, when the fake site goes to fullscreen mode, the browser flashes a warning message notifying you it has been triggered. If you miss the warning, you might think it’s a legit site.

If you want to know more about how the HTML5 Fullscreen API can be used in phishing attacks, Feross, a programmer who first discovered this, goes into great detail—and even has a working example of a fake Bank of America site—on their website.

So, now that we got all that out of the way, if you still want to disable the warning—or just reduce the time it takes to disappear—let’s continue.

How to Disable the Fullscreen Warning Message

To move forward and disable the fullscreen warning message, we’ll need to access the Advanced Preferences page, which is similar to enabling flags in Chrome.

Warning: Firefox stores every setting on this page, so you have to be careful when tinkering around here. Changing these settings can be harmful to the stability and security of the browser. You should only continue if you’re confident and sure of what you’re doing.

Type about:config into the address bar and then hit the Enter key. The page loads with a warning about the impacts of changing these preferences and the effect it can have on Firefox. Click the “Accept the Risk and Continue” button.

In the search bar, type full-screen-api.warning.timeout  into the search bar and click the pencil icon next to the result to change the value of the preference.

The number you enter is the amount of time in milliseconds until the warning starts to disappear. Keep in mind that anything under 500 barely appears after fullscreen is initiated. For security reasons, if you want to be able to see the warning when entering the fullscreen mode, you should put something around 500 milliseconds. Otherwise, enter 0. Click the checkmark when you finish.

After you change the value in the setting, you don’t need to restart Firefox. The next time you click a video into fullscreen mode, the new timeout will be used.

To revert back to the default timeout, head back to the “Advanced Preferences” page and click the reset arrow on the far right of the full-screen-api.warning.timeout setting.

As mentioned before, there are security risks to completely disabling the warning message as some known phishing scams can prey on this vulnerability. While this threat still exists, it’s probably wise to keep a timeout delay that notifies you when anything triggers fullscreen mode in the browser.

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Brady Gavin has been immersed in technology for 15 years and has written over 150 detailed tutorials and explainers. He's covered everything from Windows 10 registry hacks to Chrome browser tips. Brady has a diploma in Computer Science from Camosun College in Victoria, BC.  
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