The story of 64-bit Firefox on Windows is a long and complex one, and it is just about to end with Mozilla moving from offering 32-bit Firefox as the default download option to offering 64-bit Firefox on Windows by default.
Mozilla Firefox is offered as a 32-bit and 64-bit version on Windows, with 32-bit still the default right now when it comes to downloads.
While the 32-bit version works well, and it is the only option for systems without a 64-bit processor, it is the 64-bit version that users should consider using if their device is equipped with a 64-bit cpu.
The reason is simple: more RAM becomes available per process that Firefox uses, and 64-bit applications benefit from security features that 32-bit applications don't.
The one downside that may have kept Firefox users from switching to 64-bit was limited plugin support in the 64-bit version of the browser. It only supports Flash and Silverlight. That restriction is still there, but with Mozilla throwing out NPAPI plugin support soon -- with the exception of Flash -- that is no longer an argument if you want to stay with a recent build of the browser.
Chance is, that you are still running a 32-bit version of Firefox as you'd have to get out of your way to grab the 64-bit installer from the Mozilla website.
Back in July 2016, only 1.7% of Firefox users on Windows used a 64-bit version of the browser. That's not much. The number has probably gone up til then, but it is likely still low due to Mozilla prioritizing the 32-bit installer over the 64-bit currently.
I explained how to upgrade from Firefox 32-bit to Firefox 64-bit here, and suggest you check out the guide for a full rundown on how to do that.
Good news is that it is super easy to upgrade Firefox from 32-bit to 64-bit. All that is usually required is to download the dedicated 64-bit installer from Mozilla, and run it. All your shortcuts, profiles, bookmarks, modifications and so on will continue to work.
Note: The 32-bit version is not uninstalled automatically. I suggest you keep it around until you have worked with the 64-bit version for a while. Once you are confident that there are not any issues, you may remove the 32-bit installation of Firefox from your system.
First thing you do, is check if you can update Firefox to 64-bit. USe Windows-Pause to open the System Control Panel applet. Find the "system type" listing on the page, and check whether it says 32-bit or 64-bit.
You need a 64-bit processor. If your system does not have one, you are stuck with 32-bit Firefox. Don't worry though, Mozilla won't end support for 32-bit, but will just focus on distributing 64-bit Firefox over 32-bit in 2017 and later.
Second thing that you do is back up the Firefox profile folder. Type about:support, click on the show folder link to open it on your system.
Note that this opens the profile that is in use at the moment. Go up two directories, so that you are in the main Firefox directory under the user folder.
Select profiles, press Ctrl-C to copy it to the clipboard. Now browse to another folder on your computer, and use Ctrl-V to place a copy of it in it.
Make sure you pick the 64-bit version for Windows. It is indicated by a 64-bit icon on the download icon.
Make sure you close Firefox before you continue.
Run the installer afterwards, and follow it through to the end. Firefox will be upgraded to 64-bit. You can verify that using the method mentioned above.
Now You: Do you run a 32-bit or 64-bit version of Firefox?
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