By default, Thunderbird does not provide much information about the junk/spam mail management process. JunQuilla adds a number of features that make management of junk messages easier. A percentage match column allows you to see which messages are almost junk. An extended junk status column allows you to see why messages were classified a certain way - which might be, for example, by addressbook whitelisting, manually by the user, or by the statistical filter. And an "Uncertain" search folder shows messages that you should really classify manually, which also helps to improve the accuracy of the junk classification process.
JunQuilla is part of the MesQuilla series of Mozilla Messaging addons by rkent.