No one's saying you can't have Chrome installed.

It's really no big deal to open it when you want to watch any of those. I've never heard anyone complaining about switching from your phone's browser to the Netflix app on your phone, so I don't see why opening another browser on your computer is a big deal.

So what's the use case for having Firefox installed at all if Chrome is installed? What can Firefox do that Chrome can't?

Container tabs. Good text rendering. Support for some niche css features, the WebExtension implementation can support more advanced extensions (sidebar, runtime theme modification, dns, etc). shift+right click to bypass websites that block context menus. if a website uses the background-image css property, the firefox context menu will have the correct image options and chrome won't. i can hold ctrl and properly select HTML tables to paste into spreadsheets.

Does anyone here only have a single browser installed on their computer? I always install Chrome, but do less than 0.1% of my browsing in it. What's the reason you couldn't have both? Storage?

What you gain from using something other than Chrome is privacy, and using something other than Chromium makes it harder for Google to gain more control of the internet.

That being said, I find both Firefox and Safari to be better experiences than Chrome.

Firefox can provide a browsing experience that isn't designed by Google.