I didn't know how to use the junk mail feature in Mozilla myself until recently. I do the best of my ability!
Just clk the 'Junk Mail Icon' on every junk email. Within a short time, Mozilla will learn to delete every one for you.
Goto: mail> tools> junk mail controls. Set it to delete junk. Then delete the 'trash' once in a while.
All good things come to an end, but MAN! I hope Mozilla lives for a long while.