Remove a site's cookies from Firefox
The following steps can be taken to remove the cookies associated with a
particular website in the Firefox browser. Note: these steps were taken on
Firefox 50.0.2 on a Microsoft Windows 10 system, but should be applicable to
other versions as well.
Click on the 3 horizontal bars at the upper, right-hand corner of the Firefox
Click on Options.
In the about:preferences window, click on Privacy.
Under the History setion of the Privacy window, click on
"remove individual cookies".
In the Cookies window, scroll down to you find the relevant site.
Click on the relevant site to select it, then click on the Remove Selected