Mounting remote folder /remote/folder on hostB to local folder
/local/folder is done using:
$ sshfs username@hostB:/remote/folder/ /local/folder
If hostB is only available by tunneling through hostA, the following
sequence of commands should do the trick.
$ ssh -L1234:hostB:22 -N -f hostA
$ sshfs -p 1234 username@localhost:/remote/folder /local/folder