I attempted to install The Lightning plugin for Thunderbird. I do this by going to Tools -> Add-ons. I search for Lightning and click "Add to Thunderbird". Then I get a popup asking me if I want to open or save this .xpi file. If I click open, then Thunderbird opens a new email address with the .xpi file attached. If I click save, then try to double click it, it again opens as an email in Thunderbird. How do I install this dang thing?!
I attached two screenshots for clarification.