Firefox has a full screen mode and most, if not all media players you find online have one too. It may seem like a harmless feature and it is for the most part but there are still exceptions. This is why when you enter full screen mode, you get a warning from Firefox. It’s there for a reason but you can get rid of it if you want. Here’s how you can disable the full screen warning in Firefox.
Disable full screen warning
Open Firefox and in the URL bar, enter the following. Tap enter, and accept the warning that you see.
This will take you to the Firefox preferences page. In order to disable the full screen warning in Firefox, you will have to change several different preferences. The list of the preferences you need to change is below. To find them all, enter ‘full-screen’ in the search bar, and tap enter.
All these preferences have some sort of numerical value set for them. Some have two values such as the ‘full-screen-api.transition-duration.enter’ preference which is set to 200 200. Replace it with 0 0. You need to change all these values to zero. Where a preference has two values as in the example, you will need to enter zero twice. For single values, you only need to enter zero once.
Once you’ve changed these preferences, you will no longer see the warning you get when you switch to full screen in Firefox. These preferences collectively control different aspects of the full screen warning such as how long it remains on the screen, and how long after entering full screen it will display it.
If you want to enable the warning again, you will need to set the value for each preference back to what it was. Granted, it’s unlikely you will remember what the values were but resetting a Firefox preference is pretty easy. Right-click it, and select Reset from the context menu.
If you’re wondering why a full screen warning is necessary, it’s because a malicious website may force itself to run in full screen mode. An unsuspecting user may think something is wrong with the browser since all controls disappear in full screen mode and that’s what a website looks to exploit. A lot of malicious websites rely on users being unaware of certain features or being naive enough to give them free access to run scripts and install add-ons or just show notifications.