Hi Firefox214, glad to see you in Mozilla Support Forum.

There are two solutions for your problem:-

First is:-

In the developer toolbar you can see if there is a cookie with localhost stored as well with : cookie list.

Second is:-

Firefox supports IPv6 by default, which may cause connection problems on certain systems. To disable IPv6 in Firefox:

1. Type about:config in the address bar and press Enter.
2. A warning page may appear. Click Accept the Risk and Continue to the about:config page.
3. In the Search field, enter network.dns.disableIPv6.
4. In the list of preferences, click the Toggle button next to network.dns.disableIPv6 to set its value to true.

Firefox can't load websites but other browsers can

If it doesn't solved your problem let us know in reply section. Thanks for raising your question in Mozilla Support Forum.

There is no cookie for localhost shown in the developer toolbar. Disabling IPv6 did not solve the problem.

Can I manually create the required cookie? If yes, how? Can I make this cookie "permanent"?

Dumb question: Will "Cookie Auto Delete" addon remove the cookie I create?

To reproduce the problem:

Enter "http://localhost/" (no quotes) in search bar & then "Enter Key" which creates the error message. Open Web Developer toolbar shows no localhost cookie. Click the "+" in the Developer toolbar to manually add the cookie & nothing happens (probably operator error).

Thanks John