SiteDelta does not reliably work with newer versions of Firefox (especially with multi-process enabled). Try SiteDelta Highlight and SiteDelta Watch instead.
This extension 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 allows you to add scan or ignore regions and manage pages known to SiteDelta.
Furthermore SiteDelta can regularly check its known pages for changes and inform you whenever one of them was updated.