From MozillaZine Knowledge Base

This article applies to Firefox and Mozilla Suite.

Many websites require cookies to function properly, and block you from entering if they can't set cookies. This article will help you make sure that cookies are working.

Application settings

Firefox and Mozilla Suite/SeaMonkey have settings to enable or disable cookies. Make sure your Mozilla application is set to allow cookies from the site.

• Firefox: Open Tools -> Options -> Privacy and make sure "Accept cookies from sites" is checked and that the site you're trying to access isn't listed under "Exceptions".
• Mozilla Suite/SeaMonkey: Open "Edit -> Preferences -> Privacy & Security -> Cookies" and make sure "Allow all cookies" is selected and that the sites you're trying to access isn't listed under "Cookie Manager -> Cookie Sites".

Security software settings

Internet security and privacy software may have features that block cookies. The Zone Alarm Pro software firewall includes a "Cookie Control" feature. If you're using Zone Alarm, double-click its icon in the system tray in the lower right corner of the screen. With ZA open, click on the "Privacy" button in the left hand part of the screen. Then click on the "Main" tab at the upper right. Once you're in, turn off "Cookie Control." Similar instructions apply to the CA Personal Firewall, which also includes a "Cookie Control" feature. [1]

After changing the setting that controls cookies in your security software, clear the cache in your Mozilla application:

• Firefox: Click "Tools -> Clear Private Data". A "Clear Private Data" dialog should open (if it doesn't, click "Tools -> Options -> Privacy / Private Data -> Settings"). Check "Cache" if it isn't checked already, then click "Clear Private Data Now".
• Mozilla Suite/SeaMonkey: Click "Edit -> Preferences -> Advanced -> Cache - > Clear Cache".