Open Mozilla Firefox.
Click the three lines in the upper right-hand corner. It is right below the exit X.
A menu will appear. Click option, or preferences if you are on a Mac. It will be located eleven options down in about the middle of the menu.
On the left side of the screen, there will be a menu. Click Privacy & Security.
Under Cookies and Site Data will be a Clear Data button. It is the first option. Click Clear Data.
Select the first box, Cookies and Site Data.
Click the Clear button.
To save your changes on a windows computer, close and reopen firefox. To save changes on an Apple computer, select quit from the top menu bar.
Click Clear Browsing Data.
Confirm that you want to delete cookies.