I tried installing Lightning from the Add-ons page,

This would imply Lightning didn't exist in the Add-ons Manager in the first place. Is this correct? What is the current status of Lightning in the Add-ons Manager?

nothing happened.

When you installing Lightning certainly something will happen. Please provide more details about

• what you did
• what happened
• what you expect to happen

I don't want to lose my existing task list.

You won't.

Thanks for the reply. I restarted Thunderbird once more, and Lightning has now installed and I have my tasks/calendar back! I don't understand why it disappeared - I thought it was now bundled with Thunderbird so we didn't have to install it separately. Anyway, my problem is fixed, thankyou.