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.

I've just discovered I have this message in Bitdefender as well.

Since the last access was 18 min ago and I rebooted about then, it probably happens when TB starts up

I've seen it as well. It's normal and nothing to be concerned about.

It's likely there are built in sections of code that allow use of microphone in Thunderbird. For example some people may need to dictate to create an email etc. The check would occur on startup.