I observed this when upgrading Firefox for the first time after upgrading Ubuntu from 19.10 to 20.04. I saw no such prompt when upgrading Firefox on Ubuntu 20.04 which had been upgraded from 19.10 several weeks previously and had presumably already seen several Firefox upgrades.
This seems similar to bug 1830096
I observed this when upgrading Firefox for the first time after upgrading Ubuntu from 19.10 to 20.04. I saw no such prompt when upgrading Firefox on Ubuntu 20.04 which had been upgraded from 19.10 several weeks previously and had presumably already seen several Firefox upgrades.