I’m trying out Thunderbird with Lightning as my calendar application, but for some reason Lightning did not show up under Add-ons in Thunderbird. Instead I installed it by using the xul-ext-lightning package. However, this ended up with all options in Lightning grayed out. To fix this I had to install the libstdc++5 package as well, as reported in this forum thread.
This is strange, since it worked fine on a laptop i tried to install Lighting on just a few days ago. But in any case, it works now, so if you want Thunderbird with Lightning on Ubuntu and are having trouble installing it, you might want to try this command in a terminal:
sudo apt-get install thunderbird xul-ext-lightning libstdc++5
That should do the trick.