Install Firefox on Windows 10.
Go to Settings > System:
Click on Default apps > Web browser and select Firefox:
Confirm that Firefox is now your default browser:
Launch Firefox and right-click on a blank area next to the open tab to turn on the Menu Bar, which is NOT turned on by default (are you listening, Chris Beard?):
Go to Tools > Options:
Under the General tab, make sure that Firefox stays as your default browser:
You must do these steps when you first install Firefox or you upgrade your computer to Windows 10. There is nothing sinister, underhanded or malicious about this process. You just have to be smarter than the software.