The extension are no longer updated. Last version, 0.13.2.1, released in March 2013, can be still downloaded from Mozilla Addons.
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.