We really appreciate that you want to use the FireBox, however the last supported OS for the FireBox was Mac OS X 10.7.5. You won't be able to install it on Mojave. You'd have to go back to an older Mac running 10.7.5 to use it.
We would strongly recommend updating to a newer solution such as a Studio Series interface.
We can register your product for you, just log a support request through your https://my.presonus.com
account and click where it says "Create a Ticket".