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.