I think the search you described is Global Search ("Edit" > "Find" > "Global Search", or Ctrl + K), and the behavior you described is explained in the "Search terms" section in the following link: Global Search https://support.mozilla.org/en-US/kb/global-search If you do not want this behavior, you can use the following two search methods: 1. "Edit" > "Find" > "Search Messages" (Ctrl + Shift + F) Reference: Using Saved Searches https://support.mozilla.org/en-US/kb/using-saved-searches Although the above link talks about Saved Search rather than Search Messages, I think the concept applies to Search Messages to some extent. 2. Quick Filter (Ctrl + Shift + K) Reference: Quick Filter Toolbar https://support.mozilla.org/en-US/kb/quick-filter-toolbar