Do you see two Firefox entries when you try to change your default browser?
If you try to change your default browser and you see two entries for Firefox, chances are you have installed the browser two times! How is that possible?
It is, if you install both the 32 and the 64bit versions, perhaps performing a manual setup and having chosen the 32bit while you already use the 64 bit version.
Nothing to worry! Just go to the Control Panel and uninstall the 32bit version. Your system now shows that you have just one Firefox browser, which you can also choose to be your default browser.