Under Ubuntu, if are you using the latest version of Thunderbird from the PPA (rebranded as Shredder), you may encounter some problems with the Lightning plugin, especially if you are using the 64 bit version.
For example, with Shredder 3.0.6 you have to use the Lightning plug-in version 1.0b1 and not the latest 1.0b2.
For the X86_64 [...]