Ask a manager or higher-up in private if you can discuss the firing. Even if they have a policy of not discussing why someone is let go, you can certainly voice your concerns you have about your own job security, how your coworker's departure will affect your workload and the company's bottom line, etc.

Points you may want to seek clarity on:

- Was the fired coworker given advance notice?

- Was he given a chance to change things to avoid the firing?

- Was this decision due to technical reasons or a "culture clash"? (You may not get an answer here)

- Was he offered a severance package, or at least paid for his last 2 weeks and unused vacation time?