SiteDelta is an Add-on for old versions of Mozilla Firefox (before the
introduction of WebExtensions).

SiteDelta can scan for changes to a website since your last visit.

It adds a small Delta icon to your browser status bar. When you click onto it,
it highlights all changed text contents of the current page (since the last
time you clicked onto it). Clicking again cycles through the changes.
Rightclick onto the icon provides access to other features offered by
SiteDelta.

For current versions of Mozilla Firefox и Google Chrome , SiteDelta Watch
is the official successor.