If you're not going with Windows, then a great OS is Linux. It's been around a long time, is open source, rock solid, very secure.
I haven't installed it in a few years, but when I did I used Ubuntu and it just...installed without a hassle.
As for browsers, ppl are beginning to move away from Firefox because they've made some business decisions that have upset users.
Options I'm using now:
Waterfox
Vivaldi