Rated 4 out of 5 stars
Good add-on. I like it over update scanner for two reasons:
1) You can monitor part of a page, so you get less false positives by only checking the content you're interested in.
2) It doesn't use your bookmarks to store what it is monitoring. Update scanner does this and I found it problematic with syncing bookmarks between browsers and because I you lost bookmark specific settings when exporting/importing/restoring your bookmarks.
One feature request: Have the toolbar icon give some visual indication if any of the pages it is monitoring have changed. Currently you have to look at the sidebar to find this out if you where not at your computer when the alert occurred.