SiteDelta was an Add-on for Mozilla Firefox 4 or higher, allowing to scan for changes to a website since your last visit and to regularly check pages for changes.
New in 2017: SiteDelta goes WebExtension and becomes SiteDelta Highlight and SiteDelta Watch.
SiteDelta Highlight for Firefox
SiteDelta Highlight for Chrome
SiteDelta Highlight allows to highlight changes made to a website since the last visit.
It appears as a small icon to your browser toolbar.
When you click onto it, it allows to highlight all changed text contents of the current page (since the last time you clicked onto it).
Clicking again cycles through the changes.
Additionaly, you can add scan or ignore regions.
As advanced features (that require additional permissions), you can configure SiteDelta to automatically highlight the changes whenever
you visit a known page. Additionally, you can tell SiteDelta to check if any of the known pages has changed.
If you want to be notified about changes without opening a page or manually requesting a check, try SiteDelta Watch.
Of course, you can also install both extensions at the same time.
SiteDelta Watch allows to regularly monitors a website for changes.
It appears as a small 
icon to your browser toolbar.
When you click onto it, it allows to enable watching the current page, and will then inform you whenever the watched page is updated.
For changed pages, it highlights the changes made to the page and allows to cycle through the changes.
Additionally, you can add scan or ignore regions to define which parts of the page you are interested in.
If you only want to know if a page changed when you visit or manually check it, try SiteDelta Highlight, which works
better on pages that use JavaScript. Of course, you can also install both extensions at the same time.