The default browser is Firefox. But you can use any browser as your default browser.

As an example, let's install brave.

yay -S brave-bin

Then open the ML4W Dotfiles Settings app with SUPER+CTRL+S, open the waybar tab, scroll down and change the browser to brave.

Then execute the following command in a terminal:

xdg-settings set default-web-browser brave.desktop

To change the Waybar quicklink for the browser follow the instructions here https://github.com/mylinuxforwork/dotfiles/wiki/Waybar#define-your-quicklinks