It is possible to execute JavaScript in the parsed RSS feed when RSS feed is viewed as a website, e.g. via "View -> Feed article -> Website" or in the standard format of "View -> Feed article -> default format". This vulnerability affects Thunderbird < 52.5.2.
https://www.mozilla.org/security/advisories/mfsa2017-30/
https://www.debian.org/security/2017/dsa-4075
https://lists.debian.org/debian-lts-announce/2017/12/msg00026.html
https://bugzilla.mozilla.org/show_bug.cgi?id=1411716
https://access.redhat.com/errata/RHSA-2018:0061