From MozillaZine Knowledge Base
Many websites require cookies to function properly and will block you from entering if they can't set cookies. If websites report that cookies are disabled or if you are unable to log into websites, try the following:
Firefox and Mozilla Suite/SeaMonkey have settings to enable or disable cookies. Open the Cookie Manager and make sure your Mozilla application is set to allow cookies from the site, as follows:
- Firefox: Open "Tools -> Options -> Privacy". In Firefox 3.5 and above, select "Use custom settings for history" in the drop-down menu, if another option is shown. Make sure "Accept cookies from sites" is checked and, in Firefox 3.0 and above, make sure that "Accept third-party cookies" is also checked. Open the "Exceptions" list and remove any entries for the site, if found, with the status "Block".
- Mozilla Suite/SeaMonkey: Open "Edit -> Preferences -> Privacy & Security -> Cookies" and make sure "Allow all cookies" is selected, then go to the Cookie Manager, click the "Cookie Sites" tab and remove any entries for the site, if found, with the status "site cannot set cookies".
Security software settings
Internet security and privacy software may have features that block cookies.
- Some versions of the ZoneAlarm firewall include a "Cookie Control" feature. Double-click the ZA icon in the system tray (lower right corner of the screen). With ZA open, click on the "Privacy" button, then click on the "Main" tab at the upper right and turn off "Cookie Control." Note: The cookie blocker and other Privacy features were removed in ZoneAlarm 2010 (version 9).  
- CA Internet Security also includes a "Cookie Control" feature, described here.
After changing the setting that controls cookies in your security software, clear the cache of stored websites as follows:
- Firefox 3 and above: "Tools -> Options -> Advanced -> Network / Offline Storage -> Clear Now"
- Firefox 2: Tools -> Options -> Advanced -> Network / Cache -> Clear Now"
- Mozilla Suite/SeaMonkey: "Edit -> Preferences -> Advanced -> Cache -> Clear Cache"
If you have checked your cookie settings and you are still having problems, try the following:
- In your Mozilla application, open the Cookie Manager (see above) and remove the stored cookies associated with the problem websites (or remove all cookies) and then clear the cache. When you return to the problem website, a new cookie should be set.
- If you still have problems after doing the above, the cookies file may be corrupt. You can remove the cookies file so that a new one can be generated as follows (this will remove all stored cookies): Exit Firefox/Mozilla Suite/SeaMonkey, open the profile folder for your Mozilla application and then delete or rename cookies.txt (Firefox 2 and Mozilla Suite/SeaMonkey 1.x) and/or cookies.sqlite and any cookies.sqlite-journal files (Firefox 3 and SeaMonkey 2).