Downloaded Thunderbird 24.6.0 two days ago, installed EnigMail add-on without problems. When trying to install Lightning today, got message that version only works with Thunderbird 31. Upgraded to version 31. As a result I cannot install any add-ons: There is only a download button, but no install button. With some add-ons (Lighning) a pop-up appears, offering to save the file, with others (EnigMail, LookOut) nothing happens when pressing the download button. When saving the file Thunderbird and Windows do not know what to do with it.