13.5 will be the last Windows version, Mozilla bumped the requirement to Windows 10. We could add a warning to Windows 7 users somewhere, e.g., in about:tor. We can check the version of the OS with `Services.sysinfo.getProperty("version")`. It's `10.0` in my Windows 10 VM, and `6.1` in my Windows 7 VM. Maybe the first versions of Windows 10 also use 6.1, but if that's the case, they're unsupported too.