Try to download a full installer from https://www.mozilla.org/en-US/firefox/all/#product-desktop-release , you should be able to choose a program location there.

Please remember that user data are still inside c:\Users by default.

As far as where Firefox stores your data, by default, your profile folder will be created on the C drive. If you want to use a different drive, here's how you can set up a second profile once Firefox is up and running:

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the "Create a New Profile" button, then click Next. Assign a name like Custom, and use the button to choose the profile folder location. Important: Create a New Folder! Do not select an existing folder because Firefox will adopt all of those files into your profile, potentially leading to deleting all of those files at some point.

Finally, click the Finish button.

Firefox will switch your default profile to the new one, so you're ready to go. Click its Launch profile in new browser button and see how it looks.

To confirm the profile folder location was assigned correctly, open the about:profiles page in the new window.

Success? You can close out the old profile window and move forward with the new one.