By default C2 games cache everything to disk so they can keep working offline. See Offline games in Construct 2 for more information. I'm not sure why, but Firefox makes a prompt about this. Chrome doesn't make any prompt. If you allow it for the domain then the user will never see it again for your website.
You can take off the manifest attribute from the HTML tag like others have shown, but this means the game will no longer work offline, and could waste bandwidth from users who re-play the game a lot. I think it is just better to warn your users that the alert appears because of offline support, or ask Mozilla to remove the prompt.