The Firefox flatpak package allows access to the Downloads folder by default, so that shouldn't be a problem. The error message seems to imply that some form of authentication is needed.
Using a Chromium-based browser might do the trick though, now that (sadly) many websites are starting to treat Firefox as a second class citizen.
Here's a screenshot that shows how you can switch between Xorg and Wayland (Wayland is the option that simply says "Zorin Desktop"). Remember, you must login normally first and then logout of your account to see this button at the lower right corner.
