You can create a filter that acts on

"sender" "is not in my address book" "<name of address book>" 

so that any unknown senders would be filtered off. I'd recommend that you move these messages to their own folder.

Bear in mind that some legitimate senders do a troublesome thing where they use a random string in the left-hand part of the "from:" address. These are typically commercial messages or newsletters. These messages will not match your "friends" address book and so will inevitably get filtered out. For this reason, unless you're sure you won't get any messages of this type, you'll need to eyeball the filtered messages now and again, or set up another filter to run first that acts on just the consistent part of the sender's address, e.g.

"from" "ends with" "@example.com"
"from" "ends with" "@example.net"

etc

The "action" of this filter could be to move these messages to another folder, or just to stop filter execution. You don't want the "is not in my address book" filter to see these as they would be treated as spammy.

I have tried all of these tricks and I can tell you that maintaining a "friends address book" and a "friends filter" rapidly becomes very tedious.

I would urge you to continue marking unwanted messages as Junk, to help train your Junk Controls.

I find there are two types of messages which evade Thunderbird's Junk Controls. Some are almost entirely image and have very little text to analyze. Others have large swathes of invisible text, often hidden in comments, usually copied from recipes, travelogues, news articles etc, the effect being to dilute the few distinctive spammy words in the message with a large number of normal innocent non-spammy words, thereby keeping the message's spam score low.

Thank you Zenos - I will try this - it does sound tedious but when I get over 300 e-mails every day and only a few wind up in the Junk folder I've got to try something!