Whilst it's possible to set a start and due date for Tasks in Thunderbird, it is not currently possible to set a specific time. This is not only quite an important feature for a modern To-Do list, but is also an issue when syncing Tasks with a third-party service via an extension, as start and due times cannot be viewed, used, or edited in Thunderbird. To be clear this isn't an extension issue (most operate simply by not synchronising this data) but it does make Thunderbird very limited in terms of how it can interoperate with third-party services.