Unfortunately the bureaucracy of the Brazilian Government is so complicated, that the Mozillian Government was unable to validate it.
In practice, this means that the security certificates on the websites affiliated with the Brazilian Government are considered invalid in Firefox.
You need to import them over http (not https —
If you are paranoidal, use a separate Firefox profile. or perhaps a separate browser.
Passo 1: Clique aqui para baixar a cadeia v1.
Passo 2: O browser disponibilizará opções para proceder a instalação;
Passo 3: clique no botão 'Ok';
Passo 4: Clique aqui para baixar a cadeia v2