It would be great to have at least a possibility to sync Thunderbird events and tasks with taskcoach. This is even yet almost possible but it is not automatic and gives errors; if you export the taskcoach data as icaledar and import them in TB (go to lighting menu Events and Tasks) then you sshould see the event in TB calendar. For some reason it doesnt't work in TB 57 and I have no ide if it could work with older versions or with the new 60.
Implement the sync would be easy if there would be an option in Taskcoach to export automatically every laps of time to a specified folder. Then with a cron event you could force TB to import it at least under Linux. The first step to have compatibility should be therefore to have an option to export events and tasks from Taskcoach to a folder in your Pc.

@Jaime Rojas
I'd rather prefer TaskCoach as a replacement for that poor, unsofisticated Lightning.

@Lukasz
It must be the very same data file to be useful. At Thunderbird startup, TC-extension would simply force Standalone TC (if active) to save and exit. Any attempt to run Standalone TC while extension is active, should just bring to the front the TC-extension tab inside the main Thunderbird window.

Just by copying the data file, you are able to use TC-Thunderbird on your desktop PC, standalone TC on the laptop, portable TC in a pendrive, or even send it attached to an email to yourself.

Where would such an extension store the data file? Would it have to be separate from a file used by the stand-alone TaskCoach?

I think that more than make TaskCoach as one extensión to thunderbird, exactly. Would be easiest and fastest, build an interface between both applications. Letting to user creating tasks and syncronizing them with the lightning extensión of thunderbird, and converting email messages on tasks to be managed with taskcoach.

indeed i would appreciate task-coach as an add-on to thunderbird. that would promote both projects

Then would be worth to have an evolution mail extension too.