Whonix is an OS, a modern browser of your choice could be firefox.

Whonix runs TOR in a separate VM from your browser/user space. The idea is that even if you get hacked they don't get your IP address since they can only access the internet through the gateway VM that pushes all traffic through TOR.

I always wondered why they need whonix gateway? Couldn't they just pass it through host's tor? Why do i need to run entire full blown debian just as a proxy?

Is there any reason not to use whonix?

What do I do if macOS is my host OS?

Only VirtualBox is an option but whonix says support is experimental (they don't test it at all). So... stick with it.

No.

That's indeed far better; specifically, Firefox ESR with all the calling-home features disabled in about:config and noscript with no whitelist on top of it.

Ideally you'd make sure the one responsible for going through tor isn't Firefox, too; IE at the very least a wrapper such as tsocks when running it or even better, a VM containing the browser and the entire VM connecting only through tor.