Not brilliant, but it might work: due to recent fixes to the MAPI component of TB, which affects sen

Not brilliant, but it might work: due to recent fixes to the MAPI component of TB, which affects sending from other apps, I suggest you download the full 60.6.1 installer, install over your existing setup, select TB as default during installation, restart PC after install, and see if it fixes your problem.

https://support.mozilla.org/en-US/questions/1255111