FWIW, 64-bit Firefox will be the installer default for new installs (and re-installs) on 64-bit Windows 7/8/8.1/10 starting in Firefox 53. Migrating existing 32-bit Firefox users to 64-bit will probably happen in mid-to-late 2017. Similarly, Google plans to migrate their 32-bit Chrome users to 64-bit sometime in 2017, too.

https://wiki.mozilla.org/Firefox/Win64