This will explain how to install Firefox onto a Windows 10 machine using the installer from Mozilla.
To install Firefox:
Go to the Firefox Download Page and click Download Now
Once the installer has downloaded, follow the instructions and click it to run it and start the install
As per the instructions there, click “install anyway” if you see a message about it not being verified.
You may get the dialog asking “Do you want to allow this app to make changes to this device”. Click yes as you know you are installing Firefox, and Firefox should start installing.
When the install in complete Firefox will open.
And you are done, Firefox installed.