Yeah, this is a kind of misleading headline. I can't predict the future, but AFAIK there are no plans to launch a paid version of Firefox.
There are plans to launch paid subscription services & products in a Firefox brand family. Some of services may even be independent from the browser and be separate standalone apps & products. Some of the services may even - gasp - work in a non-Firefox browser.
Think of things like Firefox Send, Firefox Monitor, Firefox Lockbox, a system-wide VPN, an in-browser secure proxy. Specific plans are still in progress, but I don't think any of them are literally "you pay for browser now"