Delete cookies for a single site

To delete cookies for one site:

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.
Click Show Cookies.... The Cookies window will appear.
In the Search: field, type the name of the site whose cookies you want to remove. The cookies that match your search will be displayed.
Select the cookie(s) in the list to remove and click Remove Cookie.
- Select the first cookie and press Shift+End to select all the cookies in the list.
Click Close to closeClose the Cookies window.

Delete all cookies

To delete all cookies stored on your computer:

At the top of the Firefox window, click on the Firefox button, go over to the History menu (click on the Tools menu in Windows XP) and select Clear Recent History....On the menu bar, click on the Tools menu, and select Clear Recent History....At the top of the Firefox window, click on the Tools menu, and select Clear Recent History....At the top of the Firefox window, click on the Tools menu, and select Clear Recent History....On the menu bar, click on the Tools menu, and select Clear Recent History....
Set Time range to clear to Everything.
Click on the arrow next to Details to expand the list of history items.
Select Cookies and make sure that other items you want to keep are not selected.
Click Clear Now to clear the cookies and close the Clear Recent History window.