This is exactly what I want, but the UI is not that good though. I suggest that it's better to put the button in right-click list instead of a pop-up. For color and other about UI, its style is inconsistent with Firefox's default style, which is the problem. Anyway, everything is up to you.
And, I hope you could implement a switch, which can enable/disable the pop-up, I don't want to see that button every time I select something.

Ciao Francesco!

It's a great addon in principle but I found a few more bugs so I hope you could fix them.
1. using the addon destroys my google calendar UI. In the week view I cannot scroll down past 17:00 and when I want to create/ edit an event I don't see all the details below the date and time (type of calender, reminder etc)
2. the youtube page is completely messed up, with different fonts types and size
3. I thing the adress/ search bar of the browser also starts looking different.

I hope you can solve this because the idea of this addon is great!

It has a bug adding a 20px padding on the top edge of every page. Please fix it.

This is a nice simple annotation tool. Hey Franc! are you on chrome also?