Third-party cookies are cookies that are set by a website other than the one you are currently on. For example, cnn.com might have a Facebook like button on their site. That like button will set a cookie that can be read by Facebook. That would be considered a third-party cookie.

Some advertisers use these types of cookies to track your visits to the various websites on which they advertise. If you are concerned about this, you can disable third-party cookies in Firefox.

If you are concerned about tracking by advertisers, see also How do I turn on the Do Not Track feature?.
Disabling third-party cookies in Firefox can stop some types of tracking by advertisers, but not all.

Third-party cookie settings are available in the OptionsPreferences page's Privacy panelPrivacy & Security panel:

In the Menu bar at the top of the screen, click Firefox and then select Preferences or Settings, depending on your macOS version.Click the menu button and select Settings.
Select the Privacy & Security panel and go to the History section.
In the drop-down menu next to Firefox will, choose Use custom settings for history.
In the drop-down menu next to Accept third-party cookies choose Never.
Note: If after browsing with this setting, you realize you can't view properly some of your favorite websites, an intermediate solution is to set it to From visited. That will allow third-party cookies coming from previously visited websites.
Close the Settings page. Any changes you've made will automatically be saved.

Third-party cookie settings are available in the OptionsPreferences page's Privacy & Security panel:

In the Menu bar at the top of the screen, click Firefox and then select Preferences or Settings, depending on your macOS version.Click the menu button and select Settings.
Select the Privacy & Security panel and go to the Cookies and Site Data section.
In the drop-down menu next to Accept third-party cookies and site data choose Never.
Note: If after browsing with this setting, you realize you can't view properly some of your favorite websites, an intermediate solution is to set it to From visited. That will allow third-party cookies coming from previously visited websites.
Close the Settings page. Any changes you've made will automatically be saved.

Disable third-party cookies

To block third-party trackers or all third-party cookies:

Click the Firefox menu button The image "fx57menu" does not exist. and select Content Blocking.
- This takes you to the Content Blocking section of your Firefox Settings Privacy & Security panel.
Select the Third-Party Cookies checkbox to block third-party tracking cookies. To block all third-party cookies, change the setting from Trackers (recommended) to All third-party cookies (may cause websites to break).

Note: If the checkbox is already selected and you see the message Your settings in Cookies and Site Data are preventing changes to Third-Party Cookies settings, click Change Cookie Settings.

This takes you to the Cookies and Site Data section of Firefox OptionsPreferences, where you can change the type of cookies blocked.

Enable third-party cookies for specific sites

Some websites may not work properly when third-party cookies are blocked, even with the Trackers (recommended) setting. To enable third-party cookies for specific websites, click the Control Center button in the address bar and click Disable Blocking for This Site.