How are you setting up the blocking? If you are using an add-on that opts you out of tracking, please note that the add-on may be re-creating opt-out cookies for the site to read.

One way to block cookies using Firefox's built-in permissions system is as follows:

orange Firefox button or classic Tools menu > Options > Privacy

With the setting "Firefox will: Use custom settings for history" you can click the Exceptions button and create blocks for the two domains.

Does that work?

You can inspect and manage permissions for all domains on the about:permissions page via the location bar.

Note that struggling with tracking cookies may not be worth the effort.

• http://en.wikipedia.org/wiki/Tracking_cookies

cor-el wrote:

You can inspect and manage permissions for all domains on the about:permissions page via the location bar.

That is really cool. I wonder whether the list of sites could be alphabetized? Hmmm...