Simple and mostly works. On pages that it works on, the add-on works as described. I will be looking for a new add-on as this is missing many features I would expect. There is not indication that a mark is placed, There is no indication as to the order of Scrollmarks. When you return to a page there is not indication that Scrollmarks have been added to the page.

Conclusion, the add-on works but there are much better
options.

Unfortunately doesn't work with PDFs, but works well with other websites!