From MozillaZine Knowledge Base
Cookies are small pieces of textual information stored by webpages on your computer. Their many uses include remembering login information and preserving the contents of your basket on shopping sites. Cookies may also pose a privacy risk as they can be used for user tracking across websites.
Overall cookie permissions are controlled in "Edit -> Preferences -> Privacy & Security -> Cookies". You can choose to block all cookies, allow all cookies, or to allow cookies for the originating website only (thus blocking third-party cookies), In SeaMonkey 2.19 and later, third-party cookies can be restricted to allow cookies from previously visited websites only. If you allow cookies, you can also select the following retention preferences:
This has a [Cookie Manager] button that opens the Data Manager to control cookie settings per-site.
Per-site cookie management
In SeaMonkey 2.x, the Data Manager lets you view or delete individual cookies for a site ("Cookies") tab and override basic cookie settings so that you can allow or block cookies for individual sites ("Permissions" tab). You can open Data Manager from the [Cookie Manager] button in Preferences "Privacy & Security -> Cookies" described above and also from the menu item "Tools -> Data Manager".
(In SeaMonkey 1.x and the earlier Mozilla Suite the Cookie Manager was a separate panel, accessible via "Tools -> Cookie Manager -> Manage Stored Cookies" and the tabs were named "Stored Cookies" and "Cookie Sites".)
Cookie permissions are controlled in "Tools -> Options -> Privacy". You may have to choose "Use custom settings for history" in the "Firefox will:" drop-down menu to see more options. You may choose to accept all cookies that websites wish to set or, in Firefox 3.0 and above, you may choose to accept or block third-party cookies that are often set by other companies who advertise on those sites.
Starting in Firefox 3, you can block third-party cookies in the user interface, by deselecting (clearing) the option, "Accept third-party cookies".  With Firefox 22, a new option is introduced to block third-party cookies from web sites which you haven't explicitly visited before, thus making it harder to employ cross-site user tracking based on such cookies.  The drop-down menu has three options:
As well as the basic permissions above, you may also make blacklists and whitelists using the Exceptions list. The settings in this list override your basic cookie settings, so that you can have stronger control over individual sites that you have manually added to the list. You can choose to allow an individual site to set cookies, or to set "session" cookies which are deleted once you have closed Firefox, or to stop it from setting cookies at all.
Websites report cookies are disabled
Some websites will not work properly or will display an error message if cookies are disabled. If you have set your Mozilla browser to accept cookies but websites are not allowing you to log in or report that cookies are not enabled, are blocked, or are being rejected by your browser, see if the site is shown as blocked in the Firefox "Exceptions" list (under Tools -> Options -> Privacy") or in the Mozilla Suite/SeaMonkey "Cookie Sites" list (under "Tools -> Cookie Manager -> Manage Stored Cookies") and, if so, remove it from the list. For more information, read Websites report cookies are disabled.
Mozilla Suite and SeaMonkey
To remove all cookies go to "Edit -> Preferences -> Privacy & Security -> Cookies -> Cookie Manager -> Stored Cookies (tab)" (or use "Tools -> Cookie Manager -> Manage Stored Cookies") and click "Remove All Cookies". To remove specific cookies, select a cookie from the list and click "Remove Cookie".
To remove all cookies, go to "Tools -> Options -> Privacy". In Firefox 3.5, select "Use custom settings for history" in the "Firefox will:" drop-down menu, if another option is shown. Select "Show Cookies..." and click "Remove All Cookies". To remove specific cookies, select a cookie from the list, and click "Remove Cookie".
To selectively remove more than one cookie in the above dialog use the "Ctrl" key to add additional lines to the selection, and/or the "Shift" key to extend a selection, then "Remove Cookie". There is a Search bar in the dialog that you can use to search on any string within the cookie, such as a site for example, which would be the preferred method and then selectively remove unwanted cookies from the shortened list.
Sometimes removing the cookies in the Cookie Manager is not sufficient and you need to delete the file that stores cookies in your Firefox profile folder ("cookies.sqlite" in Firefox 3 and above, "cookies.txt" in Firefox 2 or below). See the section, Where are cookies stored (below) for more information.
Where are cookies stored
Cookie information is stored in the profile folder, in two files. Starting with Firefox 3.0 and SeaMonkey 2.0 the cookie information is stored in the files cookies.sqlite and permissions.sqlite. In Firefox 2 or below and Mozilla Suite/SeaMonkey 1.x, cookies are stored in the cookies.txt file and cookie site permissions are stored in the hostperm.1 file.