It's no longer there because the code was refactored to work better (or at least differently). As far as I am aware, there is still a simple function that one can override to do what you want. It just has a different name. The onus is on add-on developers to update their add-ons when new major versions of Thunderbird are released.
It's no longer there because the code was refactored to work better (or at least differently). As far as I am aware, there is still a simple function that one can override to do what you want. It just has a different name. The onus is on add-on developers to update their add-ons when new major versions of Thunderbird are released.