Which search filter are you using? Quick Filter bar is useful to search on the currently selected folder. You can filter search by 'Sender' 'Recipient' 'Subject' or 'Body' or any combination. See more here: * https://support.mozilla.org/en-US/kb/quick-filter-toolbar Global search will allow searching by messages mentioning or email address. If you enter only three letters it seems to only offer the email address, but also offers 'messages mentioning' if four or more letters in the search. You can then use the additional filters to refine the search. More info here: * https://support.mozilla.org/en-US/kb/global-search You can also search specific folders or whole accounts using: 'Menu icon' > 'Find' > 'Search Messages' There are various options which you select from drop down list eg: 'Subject'. You can save these results as a Search folder if required. Various search options can be set: * https://support.mozilla.org/en-US/kb/configuration-options-search