Checked through my junk-mail folder this evening, which I suppose I've been a little lazy about lately. There were 9 false-positives (that is, real emails classified as spam)! They included 3 from Shane, 1 from Bryan, and 1 from Rosey. All of them are on the white-list (they're in my address book, so should not be subjected to filtering at all). The others were all from Lua's account that wasn't in the address book, and they were short enough that classifying them as spam was probably correct.

Only one of the false-positives (an email from Shane) was marked with "Mozilla thinks this is spam", the rest of them were just moved to the spam-folder. That movement-without-marking seems to happen a lot (lots of spam wasn't showing the spam-recycling-box icon either)... maybe it's time to re-install?