Yes, Senator Ron Paul is correct that "Someone" should have been fired after 9/11. The attacks that were carried out against the United States had many points that the attackers should have been caught, and yet no one seemed to take the attacks serious, if indeed they had heard about them at all, at least, not until it was too late.
The US Constitution has various provisions against ex post facto laws and putting people in jail for crimes without any evidence, and this is further bolstered by the Federal Rules of Evidence. There is no evidence of any US citizens committing wrong-doing in association with the terror attacks of September 11th. Even if they were, the punishment would be death rather than prison.