Hello, if you need to access your ftp accounts via Mozilla, you just need to use such sting in your browser:
ftp://login@domain.comassword@domain.com/
where login is the login you've used during ftp account creation, password - password for this ftp account, domain.com - is your real domain name.
Also, you client can try Mac-based ftp clients, for example, Cyberduck, they work in the same way as PC-based clients.