Firefox won't launch on non-admin accounts on macOS Golden Gate

I updated my Mac to macOS Golden Gate. Tried to update Firefox from a non-admin account. It failed. Went to admin account, deleted Firefox and reinstalled from a new dmg download. All fine in the admin account. But the non-admin account (where I work day to day) won't even launch a Firefox window. It asks for admin access to install a helper. I can't see why it needs this but have given it via username and password, but it still doesn't launch. The dock icon says "Running in Background" when I hover over Firefox and the icon bounces. But no browser window