Answers do not abound, so I hope this helps. According to https://developer.mozilla.org/en-US/docs/Mozilla/Command_Line_Options that command line has not worked since Thunderbird V2. But it must be possible as they are distributing Lightning with Thunderbird 38 in May. Looking through the bugs, even the developers require a restart of Thunderbird. https://bugzilla.mozilla.org/show_bug.cgi?id=1130852 Looking around, this appears to be the best solution. http://forums.mozillazine.org/viewtopic.php?f=39&t=2550341 "For previous versions, we've installed both Thunderbird and Lightning via GPO--install Thunderbird, then copy contents of the lightning .xpi to c:\program files\mozilla thunderbird\extensions\{lightning-extension-id}\." Although I have issues with the guys location. But I have not experience, only a distaste for using the program files folder. I would have said it would be more like C:\Users\Matt\AppData\Roaming\Thunderbird\Profiles\it93nyg2.default\extensions\{e2fda1a4-762b-4020-b5ad-a41df1933103} Alternatively using the registry might be the answer https://developer.mozilla.org/en-US/docs/Adding_Extensions_using_the_Windows_Registry

Thank you very much for the info, I hope to help me