Rated 5 out of 5 stars

I recently learned about this new API
https://github.com/thundernest/addon-developer-support/wiki/Using-the-WindowListener-API-to-convert-a-Legacy-Overlay-WebExtension-into-a-MailExtension-for-Thunderbird-78

which is meant to ease the migration of legacy addons to TB 78.
Maybe this is helpful for you...

Hi, I've seen the API, but even with the API it requires a significant amount of work, because of the backwards compatibility I had and the tight integration with Thunderbird with the UI and Options.
I'm working on it…

Rated 5 out of 5 stars

A great addon.
Will it also be available for the upcoming Thunderbird 78 ?

This review is for a previous version of the add-on (0.10.8). 

I haven't started converting Mail Redirect for Thunderbird 78, as it involves a major rewrite, and much of the functions and UI I need were still changing. Now that 78 is in Beta and support for WebExt Experiments has become somewhat clear, I probably could start rewriting, but I won't be anywhere near finished when Thunderbird 78 is released.