I'd try again to remove Lightning via Add-on manager and restart Thunderbird. It should be gone then. Now reinstall Lightning, and restart Thunderbird again.

Which is what I did when it failed during the upgrade of Thunderbird, but I can't do now. I don't have that option, as it's stuck mid-install. Please see attached screen shot. There is no remove option. It just remains like that no matter how many times I restart Thunderbird.

I see. First of all create a backup of your profile.

Then locate your profile folder via Troubleshooting Information. Help (Alt-H) - Troubleshooting Information - Profile Folder - Show Folder This will open the profile folder in Windows Explorer. Close Thunderbird. In Windows Explorer navigate to the extensions sub folder. Locate the folder {e2fda1a4-762b-4020-b5ad-a41df1933103}. This is the Lightning extension. Delete the folder {e2fda1a4-762b-4020-b5ad-a41df1933103}. Your calendar data remains untouched. There may also be a 'staged' folder, which you can delete as well.

Start Thunderbird and reinstall Lightning 4.0.0.1.

Thank you, that fixed it.

I have tried everything listed and still cannot get Lightning 4.0.0.1 to install in Thunderbird 38.0.1. I did get an error message at one point that said "Lightning could not be installed because Thunderbird cannot modify the needed file." I have uninstalled, deleted folders and everything else but still only get the add on to say that it will install after a restart. At one point it was showing that it was there but disabled. I hope someone has found the solution to this. I forgot to mention that I am running Windows 7 32 bit.

I got it to work after both following the instructions here, but only after also installing from a file rather than via the "Get Add-ons" within Thunderbird.

I tried every single solution above and then some to no avail. Then one day it just started working...go figure?