I've never mounted an ftp share at boot before but if the only problem is a timing issue of having it execute before the network itself is established I may have something to try.
Set this up in fstab like you had it before and make sure it actually works by running the following command:
If it works then create a file:
With this content:
gksu gedit /etc/network/if-up.d/fstabmount
Make the file executable:
Any script placed in /etc/network/if-up.d will be executed only after the network is up.
sudo chmod +x /etc/network/if-up.d/fstabmount
I have used this techinique in the past with samba shares.