The sites for which cookies are set do not provide a strong enough hint of which add-on it is?

I don't know of any built-in way to see which add-ons generate which requests. However, there might be some developer/tester tools that could help with that. If no one has any suggestions on this forum, you could ask on the add-on development forum here: http://forums.addons.mozilla.org/.

For which domain(s) do you see cookies added?

Note that there are extensions like DoNotTrackMe that add opt-out cookies that can't be removed permanently.

If you post a list of your extension then we might be able to give suggestions about which could be causing this.

Ok, I didn't know about that specific forum for add-ons. Thanks.

@cor-el It is a paypal cookie. There are many tools for blocking cookies, but I want more than blocking also knowing who is putting the cookie in. I want a general solution, because now is this cookie and tomorrow will be another one. Also, I add and remove add-ons over time. Thanks for your help.

Hi ezillo, it's not obvious to me why any extension would need to contact Paypal, but if it is donation supported and the extension retrieves an image of the Donate button from the server when it is initialized, that might explain the cookie.

@scher2000

Sure, it can help to look for add-ons with donate buttons. Thanks.