Postbox provides a mechanism that allows add-ons or extensions from 3rd party developers to be installed within Postbox. However, it is up to each add-on developer to support the functionality of the add-ons they author. Postbox does not provide support for add-ons developed by third parties.
Unmodified Mozilla add-ons do not work within Postbox at this time. Developers of existing Mozilla add-ons need to port their add-ons to Postbox. Sometimes this can be as simple as changing a few parameters, such as product, version, etc., other times the porting effort may be more complex.
In some instances, such as Lightning, Postbox has performed the necessary porting work to get an add-on to operate within Postbox. We do this so people can get an early feel for how key add-ons will function within Postbox. But Postbox does not support these add-ons, again, because they were not developed by Postbox.