Hello Emilio, in the past I've also built the Thunderbird packages for jessie(-security) suite which is now covered by LTS.
I can easily rebuild and upload the packages for jessie and Thunderbird now too but I'm not familiar with the needed steps and things inside the LTS team workflow. Guido gave me some hints but I guess it better to ask and clarify. I'm preparing right now the packages for stretch-security and will upload them over the weekend to security. So do you have an opinion on how to continue with Thunderbird for jessie-security? I'm fine if you want to do the packages for LTS on your own, the git tree for thunderbird is up to date for debian/sid. -- Regards Carsten Schoenert