When surfing the net, websites will leave little bits of information, known as cookies, stored on your PC. These cookies allow your browser to remember your login details or site preferences. It's very easy to enable and disable them. Here's how to do it in Firefox 5.
Ensure your login-details are stored by your browser
From the Tools menu at the top of the browser window, select option.
Click on the privacy tab and from the drop down menu located next to the section labelled history, select Use custom settings for history.
Make sure the boxes next to Accept cookies from sites and Accept third party cookies are checked. Press OK.
At any time if you want to disable cookies, simply uncheck the boxes next to Accept cookies from sites and Accept third party cookies.