It depends on how the firewall is configured on the server / gateway / wherever.
Most of the time, firewalls block incoming traffic, such as traffic on port 25 coming to your server. Having said that, I usually configure firewalls to only allow outbound traffic on port 25 from known mail servers.
Either way, it's really dependent on your firewall configuration, but to answer your question with as much detail as you asked it... opening 25 outbound does not equal having 25 open inbound. If you enable outbound traffic on port 25, you'll be able to send mail. If you leave it closed inbound, then you'll not be able to receive mail.
This of course depends where the firewall lives in your network topology...