Update: Every 30 min or so it is showing the notification, i feel it is more annoying than useful. I recommend you to remember the time and check yourself with out the extension.
If you use Rapidshare a lot then you may have to wait for long before you can download the actual file, this can be avoided if you are a paid user. HappyChecker is a Firefox extension that gives you a notification when rapidshare happy hours are active.
Rapidshare Happy Hours
Monday – Friday 00:00h – 18:00h (UTC+1)
Saturday – Sunday 00:00h – 14:00h (UTC+1)