- Edited
In Firefox about:preferences#privacy
, under Cookies and Site Data
heading, you'll find Delete cookies and site data when Firefox is closed
with Manage Exceptions
beside it.
Also, in about:preferences#privacy
, under History
heading, you'll find Clear history when Firefox closes
with Settings
beside it.
However, the second option is more comprehensive and since it also allows clearing cookies on exit, it subsumes the first one. I am not sure why Firefox has both option though.