There are nefarious people who abuse peoples email addresses. Providing no-one responds or clicks on any links in the email then after a while they stop as it proves pointless to them. Suggest you send an email to your friends making them aware of the situation, asking them to not repond or click on anything, just delete the email. I would aslo create and use a signature. Tell them you will always use that signature, which will help your friends identify that the email really is from you. It is hard to know if you were actually hacked or whether they obtained those email emails from another source eg: forwarded emails where the person forwarding has not had the common sense to remove email addresses before sending. As you have linux Mint, it is possible they did not get them via Thunderbird or your OS, but via eg: webmail email account. Maybe they attempted to hack via email session cookie stealing. I would start in safe mode and run scans to see if there is any malware / key logging software on your system. Then, when computer is ok, logon to webmail and change your password. then remove old password in Thunderbird, Get Messages and input the new password.