Update Scanner is a Firefox addon which detects and warns you about changes to your chosen web pages.
Installing the extension adds an icon to your address bar. When you're at a web page you'd like to monitor, right-click the icon, select "Scan Page For Updates", and Update Scanner will then regularly check for and notify you of changes.
Clicking the notification link displays the web page. Any changes are highlighted, making them easier to spot, and you can also toggle between the old and new page.
You need more control? Left-click the Update Scanner icon and a sidebar appears listing all the pages you're monitoring. If you don't want to wait for the automatic check, you can click Scan here to be notified of updates immediately.
Each page has a Properties dialog which defines how often it will be checked, and the level of change you're looking for (you can opt to ignore changes to numbers, or updates less than 10/ 50/ 100/ 500/ 1000 characters).
You can even set up a page to open any updates in a new tab. If a change is detected, an "alert" sound is played, the modified page is opened in a new tab, and all you have to do is click it.