First of all you have to understand what is cookies ?
Cookies in Internet world
A cookie, also known as a web cookie, browser cookie, and HTTP cookie, is a piece of text stored on a user’s computer by their web browser. A cookie can be used for authentication, storing site preferences, shopping cart contents, the identifier for a server-based session, or anything else that can be accomplished through storing text data.
A cookie consists of one or more name-value pairs containing bits of information, which may be encrypted for information privacy and data security purposes. The cookie is sent as an HTTP header by a web server to a web browser and then sent back unchanged by the browser each time it accesses that server.
Cookies may be set by the server with or without an expiration date. Cookies without an expiration date exist until the browser terminates, while cookies with an expiration date may be stored by the browser until the expiration date passes. Users may also manually delete cookies in order to save space or to avoid privacy issues.
Most modern browsers allow users to decide whether to accept cookies, and the time frame to keep them, but rejecting cookies makes some websites unusable.
Now i hope you understand what is cookies. If you still have doubt just google as “Define cookies”.
Cookies is mostly for storing username and password. Cookies stored as plain text or Encrypted form. If you just close the mozilla,cookies won’t be deleted(depending on websites,some high secure website will delete cookies when session come to end). If you visit too many websites ,you may forget to logout. It is gateway for hackers. Others can steal your cookies.
Sometime you may visit some private websites or blogs whatever. Others can also view which websites you visits.
There is two features in mozila for you. First one ,you may know about that. Second one, i think people know rarely about that.
Clear Recent History:
First one is clear recent history option. It will available in:
Tools->Clear Recent History.
After selecting the menu,you can view the pop up box like this:
check all option as shown in the image.
Click “Clear Now” button.
Second One:Private Browsing:
In first one ,you have to delete manually history,cookies and others. sometimes you may forget to clear histories. Those who forgetting problem, second one will be useful.
Tools->Start Private Browsing.
If you select the private browsing option,no cookies ,history will be stored in your system.