Do you like a website and its news, but unfortunately it doesn’t have an RSS feed that automatically shows you each news that it publishes?
Update Scanner is a free extension that monitors websites, so that you can be updated anytime something changes on a page, even if it doesn’t have an RSS feed.
When you create a new entry site, you just have to set the auto-scan time after which the site should be checked in order to detect any update: changes to pages are highlighted automatically.
You can also set the minimum number of characters that should change on a page before it is recorded as an update.
Update Scanner integrates itself into your Firefox sidebar, so that you can see all the sites you are currently monitoring, and scan them manually at anytime.
The add-on is free to download and use.