“Seek” adds faceted browsing features to Mozilla Thunderbird
Faceted browsing is popular in shopping or price comparison sites (e.g. zap.co.il) which allow you to narrow the list of products by selecting a feature from several types of features.
Seek does the same for thunderbird messages, allowing you to quickly find messages.
One of the downsides is that it always re-indexes the folder whenever you switch to another and come back