Did you read the ESM support notes; you mention firefox
which was one of the desktop apps that only receive support via snap
package versions (as they are the same for all releases). The deb package is not supported into ESM which was clearly stated.
https://wiki.ubuntu.com/SecurityTeam/ESM/16.04
You also mention smplayer
which is a universe
package and ended mainstream support in 2019 (3 years only guaranteed for LTS 'universe' packages if supported by a flavor team, 5 applies to packages included on your Ubuntu Ubuntu Desktop, Ubuntu Server, Ubuntu Cloud, Ubuntu Base & packages found on the ISO), so expecting it to suddenly get ESM support when it's actual support had ended two years prior to me makes no sense.
I suggest you read the release notes more, announcements, blogs about the product, and wiki pages on support so you know what's covered.
Much of Ubuntu Desktop ESM support is via snap packages which many blogs alluded to, whilst 16.04 was still within its five years, but soon to shift to ESM.
I personally have little interest in it; however as I'm part of the Ubuntu News team I tend to read most announcements, thus recall bits even if all we did was list the blog in the Weekly Newsletter.
Also note: All releases have their quirks; Canonical had guaranteed support for Ubuntu Kylin for 5 years in the likely hope of getting more Chinese users, so packages used by that flavor in 'universe' got Canonical backing for 5 years for security fixes (it wasn't many as they mostly used 'main' packages anyway).