While in the mids of my Juniper exam preparation I ran into a problem with my Apple equipment. Managing the Juniper firewall (SSG5 in this case) with SSH was not possible from OSX. The connection itself would work, but after entering the password the connection was closed by the remote host (the firewall).
Trying this from a Windows laptop (with SecureCRT) everything worked as expected.
Some searching revealed that this is an OpenSSH bug. To manage your Juniper with SSH from OSX you need to add a parameter to the ssh command (or edit the SSH config file).
Parameter to add:
-o ControlMaster=auto
e.g. ssh willem@127.0.0.1 -o ControlMaster=auto
Or add the following line to the global SSH config (/etc/ssh_config) or the user config (~/.ssh/config).
ControlMaster auto
Juniper has a knowledgebase article (KB12409) on the issue.