You might recognised by now that there is no such thing as default browser for Qubes… right?
Qubes allows you to use many different templates, those templates based on different Linux distributions… and yes, those distributions might have default browsers. But it’s nothing to do with Qubes OS
You as a Linux user free to install and use any browser available on your distribution…
You as a Qubes user free to use any of the available distributions from the supported templates…
And these are just the ‘default’ offerings. Once you have reached the ‘power user’ title, then you can install and use any distribution and any browser in Qubes OS 