It is not about security at all. They do not want to test or support old browsers. So, they set a minimum version and tell you that you need to upgrade to that.
If they only support one browser, it is going to be Chrome. Chrome has more zero-day vulnerabilities than any other project I can think of. It is not about security.
I have been using Linux since 1993. Maybe it is time to move on.
I cannot answer your question because it proceeds from an assumption I cannot related to. In my view, Linux is much easier to setup and I have immediate access to a huge software library and am immediately productive.
Installing Windows is much more of a hassle ( the licensing alone ) and, even once installed, you have a system that does nothing useful and needs much more time to install software on before accomplishing anything. Every time you turn around, it is throwing up arbitrary and artificial roadblocks.
Unless it is already installed, I personally cannot fathom why people would want to spend their time installing Windows.
He is so cool that he is reverse engineering the source and compiling from there.
If it is only available as a .deb, it is probably targeting Ubuntu specifically.
Ubuntu is a derivative of Debian and uses the same package format. Ubuntu is much more popular though and the packages are not completely compatible.
As everybody else has said, Debian is working as intended. To respond to the actual post though, Debian is working exactly as it always has.
If you think Debian used to be good, you must really love it now. It is better than ever.
Unlike in the past, the primary drawback of Debian Stable ( old package versions ) has multiple viable solutions. Other have rightly pointed out things like the Mozilla APT package and Flatpaks. Great solutions.
My favourite solution is to install Arch via Distrobox. You can then get all the stability of Debian everywhere you need it and, anytime you need additional packages or newer packages, you can install them in the Arch distrobox. Firefox is a prime candidate. You are not going to get newer packages or a greater section than via he Arch repos / AUR ( queue Nix rebuttals ).