For programmers: Source code
How to use Notify?
It's easy. After you install Notify and restart Firefox, open the Tools menu and click Notify - Monitored sites. You'll see this window:
First tab shows sites, which will be monitored. You will be notified of every change on those sites.
Add your favorite sites and see how it works.
Another way to add a site: Open it, click with the right mouse button and choose "Monitor for changes".
Second tab - Calendar.
Here you can add a site, which will be opened on a chosen day.