The machine I upgraded to Windows 10 on Saturday did not have Thunderbird installed. I did a clean install of Thunderbird 38.1.0 with a clean profile. Upon first run I noticed Lightening was already there along with some options at the bottom:
First run of a fresh install of Thunderbird 39.1.0
I like the way Mozilla has this setup that you can either keep or disable Lightning. This is what they should have done with Pocket in Firefox.
