Thunderbird: nsemail and nscopy files left in /tmp

This bug was introduced in v68.x and it is not yet fixed. After investigation here is the exact situation:

Bug detail:

Thunderbird do use ~/tmp/ns* since earlier version when sending email, but it also do delete those temporary files just after using them, while tb is still open. On v68.x a coding mistake make thunderbird try to delete those temporary files on the location /tmp/ns... instead of ~/tmp/ns... thus setting $TMPDIR variable to /tmp fix the issue. (awaiting an upstream update)

Solution:

Using the following command to run Thunderbird will fix the issue (similar to the other answer)

export TMPDIR=/tmp; thunderbird;