Does not seem to be working for Firefox 73.0.1 (Win10). I type in something in the popup box and hit save. When I reopen the box (on same webpage), I get empty box.

Finally found!
Evaluation was set to 3 in hope of improvement in the future
Expect additional sync and sidebar features

Nice! Was just about to give up looking for an add-on that would let me write notes specific to an individual web page, and then I found this, which does exactly and simply that.

Rating it 4 rather than 5 as some additional features would be welcome:
- option to use the sidebar for notes rather than a pop-out box
- some basic formatting
- sync to other browsers via Firefox account (at least I'm assuming it doesn't already do this).

Still happy to recommend it.