Hmm, [https://addons.mozilla.org/firefox/addon/thunderbird-biff/ Thunderbird Biff] may not be set up to work with 64-bit Firefox, but it's hard to know unless you see something in its documentation. It might be a coincidence, because Firefox 55 probably made some further modifications to how extensions run in both the 32-bit and 64-bit versions. What do you think about using desktop notifications in Thunderbird? Is that too much information? (In the Options/Preferences dialog, General panel, ''When new messages arrive:'' 'Show an alert') What are the other tools you referred to?