You don't understand.
It's not a matter of speed, it's a matter of time. I'll explain for you.
After leaving a game and rejoining the lobby as an unsubscribed member, there is a 15-second countdown which appears informing you of when the advert will close. After a certain amount of time, a link appears which allows you to 'skip ad'.
The strange thing is, in Chrome the link appears after 5 seconds of the 15-second countdown has passed, but on Firefox, it only appears after 10 seconds have elapsed.