User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0
Steps to reproduce:
I tried to create a WebExtension for Thunderbird that changes the new message HTML before the user starts typing in it. It should add a custom CSS style to every composed message, including replies and forwards. This feature was provided by the now dead and in the future unsupported Stationery add-on.
I read the docs: https://thunderbird-webextensions.readthedocs.io/en/68/
It doesn't contain anything about this feature. So I assume it's not possible. This is the first feature I'm looking for regarding Thunderbird add-on development. So for now this is a 0% support rate for existing add-ons' features.
I fear the day when Thunderbird 78 will break everything I was used to and that made me more productive. Maybe I'll finally switch to a web-based MUA by then.