Cookie Exception

You might not want to allow third-party cookies, for privacy reasons. See here for a discussion.

In that case, you can add a Firefox Cookie Exception.

To add a cookie exception for Yahoo Groups in Firefox, do the following:

Select the Tools Menu Item to open the Tools Menu.
Select the Options Submenu Item to open the Options Window.
If necessary, select the Privacy (Padlock) Icon from the left pane (which acts as a tab) to display the Privacy Options in the right pane.
If necessary, use the scrollbar on the right side of the right pane to find the Cookies section in the right pane.
If there is a plus-sign icon to the left of the Cookies section, click on it to open the section.
Click on the Exceptions button to open the Exceptions Window.
Type or paste groups.yahoo.com into Address of web site: textbox. This has to be exact.
Click the Allow button to allow cookies for Yahoo Groups.
Click the OK button to close the Exceptions Window.
Click the OK button to close the Options Window.