Last time I was at a company that had blackout periods for employee trading, it didn't apply to former employees. I quit on a Friday and exercised then sold all my options the next Monday while my former coworkers were still in their blackout periods. Hopefully it is the same at Twitter.
The blackout policies are a tiny part an employer policy and a large part an anti-insider trading policy. If you learned of what might be material, non-public information while an employee, then got fired/laid off/quit, that only puts you in the clear with respect to the employer's policy (what are they going to do? fire you a second time?) but does not put you in the clear with the SEC.
