A shield icon will appear in your address bar whenever Firefox blocks tracking domains.
To see which resources are being blocked, you can open the web console and look for messages under the Security tab.
Starting with Firefox version 43, you will be able to change the block list used to block third-party trackers. By default, Private Browsing with Tracking Protection uses the Disconnect.me basic protection list. You can change this to use the Disconnect.me strict protection list instead.
Tracking Protection is enabled by default when you're in a Private Browsing window. You may choose to disable Tracking Protection for a particular site or for all sites.
Since Private Browsing mode doesn’t keep any information about your browsing session, when you disable Tracking Protection for a site, it only lasts for this session. When you start a new Private Browsing session, Tracking Protection will be turned on again for all sites.
If you're already in a Private Browsing window, type about:privatebrowsing in the address bar and press the Enter key to go to the Private Browsing home page.
Click the link that says Turn Tracking Protection Off. If Tracking Protection is already turned off, you can turn it back on from the Private Browsing home page by clicking the link that says, Turn Tracking Protection On.
Clicking on the green check-marked button will turn Tracking Protection off. If Tracking Protection is already turned off, you can turn it back on from the Private Browsing home page by clicking the same button, which will look like this: