Lightning is a Thunderbird add-on, and you can't install it for Firefox. http://chrisramsden.vfast.co.uk/3_How_to_install_Add-ons_in_Thunderbird.html

This is because you were left-clicking the link and going about installing it in Firefox when it is a Extension for Thunderbird and SeaMonkey, instead of right-click and saving it to disk.

To further clarify for us ninnies: You have to save the Lightning app to disk (Tools | Add-ons | Lightning | Add to Thunderbird | Save to disk ). Then go back to the main Thunderbird page and select (Tools | Add-ons | (the little gear icon in the upper right corner) | Install Add-on from file | Lightning, etc.) | Open | Install add-on | Restart now ) If you do not have a pre-existing calendar file in Lightning, but do have one in Google, you can export the Google file, then import it into lightning. It may also be possible to directly synch with Google, but I went the Export/Import route and it worked.