Lightning is a calendar and task list add-on for the Thunderbird email app, by Mozilla. You may like the Lightning calendar a lot, though some reviewers have mixed feelings. Development cycles are a little slow, so don’t expect bug fixes quickly.
Thunderbird (mail client) is required for Lightning, and Thunderbird has a variety of add-ons that you can install to customize your Thunderbird + Lightning experience. Read our Thunderbird setup instructions for a list of add-ons we do and do not recommend.
NOT for Tasks
Don’t use the Lightning task list for anything. It is not TRO-capable, and using it can even activate a few bugs in Lightning.