User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:70.0) Gecko/20100101 Firefox/70.0

Steps to reproduce:

Upon upgrading to Ubuntu 19.10, thunderbird leaves nsemail and nscopy files in /tmp:

# ls -l /tmp/ns*
-rw------- 1 alistair alistair 5522 Nov 18 08:42 /tmp/nscopy-1.tmp
-rw------- 1 alistair alistair 8883 Nov 18 08:43 /tmp/nscopy-2.tmp
-rw------- 1 alistair alistair 8311 Nov 18 08:49 /tmp/nscopy-3.tmp
-rw------- 1 alistair alistair 3455 Nov 18 08:38 /tmp/nscopy.tmp
-rw------- 1 alistair alistair 25790 Nov 15 09:26 /tmp/nsemail-10.eml
-rw------- 1 alistair alistair 5840 Nov 15 09:28 /tmp/nsemail-11.eml
-rw------- 1 alistair alistair 25442 Nov 15 09:29 /tmp/nsemail-12.eml
-rw------- 1 alistair alistair 2838 Nov 15 09:29 /tmp/nsemail-13.eml
-rw------- 1 alistair alistair 8556 Nov 15 10:10 /tmp/nsemail-14.eml
-rw------- 1 alistair alistair 1659 Nov 15 10:32 /tmp/nsemail-15.eml
-rw------- 1 alistair alistair 3240 Nov 15 10:47 /tmp/nsemail-16.eml
-rw------- 1 alistair alistair 3260 Nov 15 10:57 /tmp/nsemail-17.eml
-rw------- 1 alistair alistair 107975 Nov 15 13:43 /tmp/nsemail-18.eml
-rw------- 1 alistair alistair 4878 Nov 15 13:44 /tmp/nsemail-19.eml
-rw------- 1 alistair alistair 21150 Nov 13 08:43 /tmp/nsemail-1.eml
-rw------- 1 alistair alistair 2974 Nov 18 08:41 /tmp/nsemail-1.html
-rw------- 1 alistair alistair 6328 Nov 15 13:45 /tmp/nsemail-20.eml
-rw------- 1 alistair alistair 134689 Nov 15 13:49 /tmp/nsemail-21.eml
-rw------- 1 alistair alistair 2733 Nov 15 17:15 /tmp/nsemail-22.eml
-rw------- 1 alistair alistair 53857 Nov 16 09:23 /tmp/nsemail-23.eml
-rw------- 1 alistair alistair 89392 Nov 16 09:39 /tmp/nsemail-24.eml
-rw------- 1 alistair alistair 80120 Nov 17 10:52 /tmp/nsemail-25.eml
-rw------- 1 alistair alistair 6361 Nov 17 18:45 /tmp/nsemail-26.eml
-rw------- 1 alistair alistair 3455 Nov 18 08:38 /tmp/nsemail-27.eml
-rw------- 1 alistair alistair 5522 Nov 18 08:41 /tmp/nsemail-28.eml
-rw------- 1 alistair alistair 8883 Nov 18 08:43 /tmp/nsemail-29.eml
-rw------- 1 alistair alistair 29937 Nov 13 09:37 /tmp/nsemail-2.eml
-rw------- 1 alistair alistair 5057 Nov 18 08:43 /tmp/nsemail-2.html
-rw------- 1 alistair alistair 8311 Nov 18 08:49 /tmp/nsemail-30.eml
-rw------- 1 alistair alistair 2015 Nov 13 09:40 /tmp/nsemail-3.eml
-rw------- 1 alistair alistair 4949 Nov 18 08:49 /tmp/nsemail-3.html
-rw------- 1 alistair alistair 183446 Nov 14 08:48 /tmp/nsemail-4.eml
-rw------- 1 alistair alistair 59817 Nov 15 08:41 /tmp/nsemail-5.eml
-rw------- 1 alistair alistair 9751 Nov 15 08:47 /tmp/nsemail-6.eml
-rw------- 1 alistair alistair 2967 Nov 15 08:49 /tmp/nsemail-7.eml
-rw------- 1 alistair alistair 2983 Nov 15 08:54 /tmp/nsemail-8.eml
-rw------- 1 alistair alistair 19780 Nov 15 09:14 /tmp/nsemail-9.eml
-rw------- 1 alistair alistair 27900 Nov 13 08:34 /tmp/nsemail.eml
-rw------- 1 alistair alistair 1317 Nov 18 08:38 /tmp/nsemail.html
-rw------- 1 alistair alistair 1115 Nov 18 08:41 /tmp/nsmail-1.tmp
-rw------- 1 alistair alistair 2424 Nov 18 08:43 /tmp/nsmail-2.tmp
-rw------- 1 alistair alistair 2000 Nov 18 08:49 /tmp/nsmail-3.tmp
-rw------- 1 alistair alistair 967 Nov 18 08:38 /tmp/nsmail.tmp

I reported this to the Ubuntu package maintainer, who asked me to report it upstream:

https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/1853007