Make browser give a custom made note when re-visiting websites

There are several Firefox extensions that can do this. FloatNotes works well.

1.) Install FloatNotes extension

2.) Restart Firefox

3.) (Optional) Configure settings Tools -> FloatNotes -> Preferences

4.) On any webpage, right-click then Create note...

5.) Type your note, then click outside the note to save

6.) Close the tab/window/firefox

7.) Return to the same page later and the note will be there and in the same spot!

If you prefer Chrome, then Note Anywhere is a good choice.

1.) Install Note Anywhere

2.) Select note button to create a new note

3.) Type your note, then click outside the note to save

4.) Close the tab/window/chrome

5.) Return to the same page later and the note will be there

Bonus:

In addition to saving notes it might also be useful to know if you've visited a link and how long ago you did (uses your history).

1.) Install Link Status Redux

2.) Restart Firefox

3.) Hover over a visited link

If the ads are presented as links, visited links are normally visually distinct from unvisited ones. If they are not so on that website, you can override its .CSS to make them distinct.

You can also modify the HTML presented by the website by using your own user script.

In Firefox, the tool to use is the Greasemonkey extension.
In Google Chrome, Greasemonkey scripts work natively.

Lots of information and tutorials can be found on the internet for writing Greasemonkey scripts.

This is as far as I can go without knowing more about this website that you describe and how the ads are presented in HTML/CSS.