The standard release of Firefox for Windows is the 32-bit release. You can optionally replace it with the 64-bit release, but if you use any plugins other than Flash and Silverlight, unfortunately those will not work in the 64-bit release.

User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Firefox/45.0

The WOW64 in your useragent shows it is a 32-bit application running on 64-bit Windows.

If you download from www.mozilla.org you should get the 32-bit version for Windows so if you want the Win64 Firefox go to the Systems & Languages link www.mozilla.org/firefox/all/

Also Windows users using 32-bit Firefox will not get a update to 64-bit (Win64) Firefox, at least probably not until 32-bit Windows version of Firefox is perhaps dropped in future. This would mean support for WinXP, Vista, and 2003/2008 servers would be dropped then.

Also you need 64-bit Windows 7, 8 (8.1), 10 to use Win64 Firefox.