In the past couple of years, there has been a new plug for thunderbird to archive emails with SuitecRM.
It is not yet at the same level a Opacus was, but it works well. It is making small, incremental improvements, and worth looking at.
Check is out:
It will fill the void left by OPacus, and while not at par with it, it will still make Thunderbird part of your toolbox when using suitecrm.
Please use it, review it and encourage the developer.
Joseph