Ubuntu itself doesn’t have an ESR package, only https://packages.ubuntu.com/focal/firefox which is at 125.
The Mozilla PPA does have an ESR package, but per https://launchpad.net/~mozillateam/+archive/ubuntu/ppa?field... it’s at 115.
<dialog> has been supported since Firefox 98, meaning ESR 91 was the last release lacking it, and it reached end of support over a year and a half ago.