Comment on Track Web Site Changes in IE, Firefox, Chrome, Opera, Safari and More by Hokator

Very nice and simple extension which I’m using for this is Wachete, you can find it for Opera and also Chrome.
Same time their web page is https://www.wachete.com
Cool thing is you can track changes on part of website, whole website or site with subpages.
They have also mobile apps with widgets on desktop.