From MozillaZine Knowledge Base
Located above the message-list pane is the quick-search box, which will allow you do searches in the currently selected folder. Just type a search term in the box and the messages displayed in the message list will be updated accordingly. Using the drop-down list, you can choose to search messages by subject, sender, subject or sender, or entire message body.
To do more complex searches, go to the "Edit" menu and choose "Find -> Search Messages...". Or, you can use the keyboard shortcut: "Ctrl+Shift+F". In the "Search Messages" dialog that pops up, you can search by various other criteria (such as reply status and message age), and you can use multiple criteria in defining your search.