File Bug Report opens your browser at bugzilla.mozilla.org on the guided bug entry form with a URL that pre-fills the build-id field with the Thunderbird or Sunbird navigator.userAgent string. You will see the value of this field in the form, where you can edit it before you submit your bug report to be stored in the bugzilla database. It may contain information about the version of Thunderbird, Lightning, or Sunbird, and may also contain information about other add-ons that add themselves to the userAgent string.