That said, there are some limitations in transitioning from 32-bit to 64-bit.
"Firefox 64-bit for Windows, by design, has limited support for plugins and users will notice that certain sites requiring plugins that worked in previous 32-bit versions of Firefox might not work in this 64-bit version. As we’ve shared previously, Mozilla intends to remove support for most NPAPI plugins in Firefox by the end of 2016," Mozilla stated in a blog post.
It's been an interesting road up to this point. Mozilla's been toying with 64-bit flavors of Firefox in Windows for nearly a decade, but none of the previous builds ever graduated out of beta. Some even stalled in alpha, and for a short period in 2012, Mozilla made the decision to stop developing a 64-bit version altogether.
More recently, Mozilla promised to deliver a 64-bit build of Firefox for Windows by the end of the year, and here it is with just over two weeks to spare.
So, what's the payoff for downloading the 64-bit build? Just like the 64-bit version of Windows, Firefox can properly address more than 4GB of RAM. Casual users aren't likely to notice a difference in performance, though in theory, the 64-bit build should allow for more tabs to be open without taking a performance hit.
Beyond that, developers should be able to build more complex web goodies, including games, though for now the biggest payoff is being able to say, "Hey, I'm running a 64-bit version of Firefox!" Awesomesauce.
The 64-bit version is introduced in Firefox 43, which also brings about Private Browsing with Tracking Protection, improved API support for m4v video playback, the ability to choose search suggestions from the Awesome bar, and a few other new features.
Be advised that n-place upgrades will keep you on the 32-bit build. To grab the 64-bit version of Firefox, go the download page and click the 64-bit download arrow next to your language of choice (or just click here for English).
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Source: http://www.maximumpc.com/firefox-browser-brings-64-bit-goodness-to-windows-in-stable-form/
