If I put a filter on an unwanted email address is that the same as blocking it so that they can't get through again? Because I've done that to a couple of addresses and they got through again or is't it foolproof?
Depends what programme your using, filters and blocking are quite different. I use filters to sort between work and personal and family folders. Filters have to be quite specific and it's easy to get one slip through the 'net'.
Firefox is an internet browser, not email client software. Are you using webmail or a program on your pc, such as Windows Live Mail, M/S Outlook or Thunderbird?
It does appear to state that currently you are not able to block messages from specific addresses or domains, but you can set the filter to send to trash and delete any unwanted emails from specified senders as they arrive in you mail account.