You won't be getting unemployment benefits if you are fired. Also professional reputation, negotiation of severance terms, getting a positive reference, rehireability.
You won't get severance if you're fired in most cases.
You won't get unemployment if you voluntarily resign.
No, you don't. Oregon Unemployment, Claimant Handbook, Section 6. Source: I helped a friend through unemployment. He was fired from work, and his employer provided a false reason for the firing, triggering denial of benefits. We looked at his options, and it would've been cost-prohibitive to retain a lawyer to fight it, which would've been required.