OP posted here from an Android device.

What "other unit"?
Are you able to download a Firefox desktop / laptop setup package from here?
https://www.mozilla.org/en-US/firefox/new/

Also specifically which Operating System would Firefox be installed on?
Does any web browser work on that specific device? You don't need to use Firefox to download the setup package, but the device does need to allow the Firefox setup executable to be able to access the internet to download the full Firefox installation program.

The device is a mobile Android 10 Galaxy 14 and it has google as webbrowser but I wanted to change it to Firefox. The Firefox app is downloaded and the Icon is visible and it is set as standard webbrowser but I do not get to the Internet, just the support page when I press the Icon.

I was asked to download Firefox on another device as the last step but my only other device is an iPad, which is too old and could not install Firefox. Is this the reason why I have not been able to finish the installation?

What is wrong!? Best regards Anders

Sync is optional.

You can close the tab once you have created the Sync account if you do not want or can't add a mobile device.