Only 1.7% of all Firefox users on Windows are running a 64-bit version of the browser while the majority of users runs a 32-bit version of Firefox according to Mozilla. Mozilla released a 64-bit version of the Firefox web browser for Windows last year but has not actively promoted it yet.
The main Firefox Stable download page on the Mozilla website offers the 32-bit version of the browser by default.
Mozilla does not highlight that on the page, and what makes things even more complicated is that you don't get an easy to click on link to download the 64-bit version instead.
Firefox users who want to download and use a 64-bit version of the browser on Windows need to search for it actively. That's the main reason why only 1.7% are running Firefox 64-bit currently.