When checking how many Yahoo cookies I have for their email and others they assign for each category on their news reader/ home page...there are 13 Yahoo cookies. I've
read several of the news items there this morning. So, if you click on any of the articles that are not Yahoo originals then you will have cookies installed from those sites as well.
Firefox has toyed with the idea of blocking Third party cookies by default. But I know some sites require allowing ads to view content or play games. The vast majority doesn't as of
You can block the install of Third Party cookies...aka...ad/ tracking cookies. Once you have blocked from installing...run CCleaner to remove the ones presently installed.
You will still get first party cookies when visiting new sites. Some of those may track you, too. Such as Amazon and Google.
How to disable third-party cookies in all major web browsers
If you haven't installed an ad blocker in Firefox or Chrome if you have that, too....then I recommend Adblock Plus...used it for many years.
NoScript is another add-on I would recommend. It takes some time to learn to use it but well worth it.
Adblock Plus :: Add-ons for Firefox
Adblock Plus - Chrome Web Store
NoScript Security Suite :: Add-ons for Firefox