Custom headers

  1. Install the about:config extension to be able to access hidden preferences. Open Tools -> about:config.
  2. Type mail.identity.id into the Filter field. A list of the settings for each identity is shown. Click on the first column header to sort by setting name. The setting names all begin with mail.identity.idX, where X is the identity number. Locate the identity you want to set the header for.
  3. Look for a preference named mail.identity.idX.headers.
    If it already exists, edit it by double clicking, and add a name for the header you want to add, separated from the previous value by a comma. For example, if there already is a value of face and you want to add an X-Face header, you might use face,xface.
    If the headers preference does not exist, right-click in the list and use New -> String to add it, entering xface as the value.
  4. Right-click in the list and use New -> String to add a new preference that will store the header value. In this example, it is mail.identity.idX.header.xface. Enter the complete header line, beginning with the header name followed by a colon and a space.
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