Update scanner is easy to setup because it is a Firefox plugin and integrates into the browser, so you do not need to setup browser. But it works only in Firefox and only when Firefox is open. You can determine the level of precision, so that minor changes do not cause alert, but you cannot choose the area of the webpage to monitor. So, it is not suitable for dynamically changing web pages.
The version for FF 46 was good. The new version has one problem that makes it essentially useless (except general public pages.) It can't pass login cookies while scanning.Reported at https://github.com/sneakypete81/updatescanner/issues/119
Probably not possible, since the background scan process doesn't share cookies with the main browsing session.
Our users have written 2 comments and reviews about Update Scanner, and it has gotten 36 likes