Do you use any addons that may require use of microphone? Do you use any speech-to-text software? Such as dictating a message or maybe used to read emails etc? I know that the address book allows people to add a telephone number. It will look like a link and if you click on it - it asks what program to use. I looked in the Config Editor and it does contain some preferences that mention microphone. Firefox would require media access, not sure if this is come code that has just been included in Thunderbird. I'll see if I can find out a bit more.