It seems like a lot of Firefox addons are not longer compatible with Seamonkey after Firefox changed their addon functionality and Seamonkey is still using a old Firefox version. The old addon system was more powerful so I guess they kept it also for backwards compatibility of existing addons. But now the new addons won’t work and the old ones are old and probably not longer maintained.

Usually all email clients have a good web version nowadays which make the integrated email less interesting.

To make this project live again it would need to break backwards compatibility and adjust to the Firefox release cycle.

Overall it would be a good candidate to integrate with AI.