Global Search
Thunderbird includes a new message indexing and search system called "Gloda", which is short for "Global database". It improves search performance, provides sophisticated full-text search capabilities and displays faceted (categorized) search results.
Global search queries all of your messages, regardless of the account the message is associated with or the folder where the message is stored. The Quick Filter, on the other hand, is used to search for messages in the current message list. (See the Quick Filter Toolbar article for information on using the Quick Filter.)