From Sunbird export your calendar in iCalendar format (*.ics). Then import that file into your Home calendar in Lightning or create a new calendar before. Events and Tasks (Alt-N) - Import

Having stuffed about I think a tad more information might be hlpful.

Having installed both Portable apps, I managed to import my older Sunbird events, but it ignored the tasks (all of which are completed, but hey, it is also about retaining history)

Anyway, I have just spent some time installing and reinstalling Firefox and Thunderbird in Windows 10. And got stuck on importing my calendar. I have not even tried email or an address book; since they only exist on another computer.

Trying the Tools, Import... option shows that this only addresses email.

So eventually I opened Events and Tasks. Guess what, another Import option. This worked like a dream, and, unlike Portable Thunderbird, this imported Tasks and Events. So I am very happy.