It could be this one. I remember the yellow highlights.

It was many years ago and definitely not a cloud service so not Distill Web Monitor. It didn't let me define fields to monitor, so not PageProbe.

I wouldn't have used a cloud service. What's the point? My browser already have everything it takes to download pages and diff them. I understand that if the computer is off the browser won't monitor the page but it was ok with my use case: if it's off I won't be able to react to the CVE.

And for other use cases, it's no business of anyone else but me to know what I'm monitoring.

I remember using the addon too. The reason for the addon had words to the effect that it enabled you to keep track of sites that did not have an RSS feed which was the more standard way getting new web content.