Hi,
I’m using Firefox 63.0.3 (64bit) under macOS Mojave 10.14.1.
The OS is determined correctly but clicking on the green button „Download for macOS“ results in nothing happening.
Since you are mentioning Chrome and Safari to be working for you I tried out Safari and this works.
So it must be as you are saying an issue with the button under FF.
Thanks. Solved at least with this workaround.