The Firefox web browser is available as a 32-bit and 64-bit application for all supported desktop operating systems including Windows.
The 64-bit Firefox version for Windows is a fledgling however when compared to the Linux and Mac OS X versions, as it has just been available officially since December 2015.
64-bit builds for Windows were available years earlier, and third-party forks like Waterfox or Pale Moon offered dedicated 64-bit versions long before Mozilla considered offering them officially on the main download hubs of the browser.
While Firefox 64-bit builds are available officially on the stable channel, the main download page still pushes 32-bit versions to user PCs.
64-bit versions of Firefox offer better security and better performance for demanding web applications.
Firefox users on Windows are running 32-bit versions of the browser basically, unless they have downloaded the 64-bit version of Firefox for Windows explicitly.
As far as stats are concerned, Mozilla did a market analysis back in 2015:
The stats will have changed since then. Firefox 64-bit for Windows has been released to the stable channel. While downloads are still hard to find, it seems likely that the availability increased the number of 64-bit Firefox on 64-bit Windows.
Most processors are 64-bit that get sold these days, and this will impact the stats as well over time.
Mozilla plans to put the focus on 64-bit versions of Firefox for Windows in the near future. The organization released a proposed schedule that highlights how it wants to accomplish that.
Note: These are proposals, and things may change along the way.
The percentage of 64-bit versions of Firefox on Windows will pick up pace once Mozilla makes the version of the browser a default option in the installer, and when the upgrade of 32-bit versions of Firefox to 64-bit begins. (thanks Sören)
Now You: Do you run a 32-bit or 64-bit web browser?
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