Here are the statistics I've collected on Mozilla's mail filtering accuracy over the last 24 hours:
Total emails received: 41
Spam emails received: 27
Non-spam emails received: 14
Percentage of incoming email that is spam: 65.9%
Spam emails correctly identified by Mozilla: 21
Spam emails not identified by Mozilla: 6
Spam recognition rate: 77.8%
Non-spam emails incorrectly categorised as spam: 0
77.8% isn't anywhere near a high a percentage as I would have expected, but it's possible that I was simply unlucky today and received some unusual spam that defied the filters. 24 hours isn't nearly enough of a sample period to draw any conclusions, so I'm going to continue this for a while longer and collect some more meaningful data... I'm intrigued to know the numbers on this.