A cookie is a custom file which is created in your computer when you access websites.
The cookie contains sensitive data, which is used to validate you and authorize you for logging in to the website and it also contains other information which you have submitted to the site.
To view all the cookies which are stored in Mozilla Firefox,
Go to tools > options.
Then go to the privacy tab and then under cookies click the show cookie button.
Now the cookies dialog box appears where you can view all the cookies and remove unwanted cookies.