Periodically deleting cookies in Safari
If you open Safari in private browsing mode, all cookies you may have are not accessible. Feel free to log in to whatever site you want and the cookies only persist for that session as you are familiar with from Firefox.

If you want to script a periodic purging of all cookies, that is trivial if you use Automator to record the action of launching safari, selecting preferences and selecting the clear all cookies button.

That's probably overkill, since private browsing does about all you ask already.