The forum itself might not use HTTPS, but the login screen (which is the same login screen used by the Mogstation and the SE Account Management System) most definetely does use HTTPS encryption (I just checked by logging out and then logging back in, taking careful note of the address bar and Firefox clearly listed the security credentials and had the 'https' at the front of the address.

Either way, your password should be encrypted as the login screen is HTTPS even if the rest of the forum isn't.