Solution for spam mail not being moved to Junk folder automatically

My junk mail used to be automatically moved to my junk mail folder. Lately it just gets marked as junk and sits in my inbox, even though I still had the option enabled. If you find that your email client (Thunderbird, Mail.app, etc) no longer automatically moves your junk mail, try this out. The directions are for Thunderbird but its easy enough for any client.