Uh, oh, you are running franken-debian – mixing the PureOS and Debian repositories.
The easiest way out of this is complete reinstall.
Even so, as Amosbatto said, firefox in PureOS amber is not the newest; it is version 68.12.0esr. So might not be worth the effort.
As to how upgrade from green to amber, this requires manually editing /etc/apt/sources.list and replacing all references to “green” with “amber”. I’m not aware of any GUI tool that would allow upgrading to a newer OS release.
How to go back to PureOS from the mix of debian and PureOS you are running – you might want to check that post of mine