While Mozilla did not cite the 64-bit support with any fanfare, it was the biggest difference between Firefox 43 and its predecessors.
A preview of the 64-bit Firefox for Windows was issued more than nine months ago, when Mozilla's usual schedule would have meant a May release.
The biggest advantage of a 64-bit browser on a 64-bit operating system -- like Windows and Apple's OS X -- is that it can address more than the 4GB of memory available to a 32-bit application, letting users keep open hundreds of tabs, and run larger, more sophisticated Web apps, notably games.
The appearance today of the Firefox 43 ends Mozilla's climb to catch up with rivals, who had offered 64-bit browsers long before, in some instances years. Google, for example, shipped a Windows 64-bit Chrome in August 2014 and one for OS X in November of that year, while Apple's Safari and Microsoft's Internet Explorer (IE) have had 64-bit editions on OS X and Windows since 2009 and 2006, respectively. Opera Software, the Norwegian browser maker known for its same-named desktop flagship, also offers a 64-bit edition on Windows.
Mozilla was the last holdout among the top five browser builders.
Its path to a 64-bit Firefox for Windows has been contorted. Although Mozilla has long had 64-bit versions for OS X and Linux, the developer shelved work on one for Windows in November 2012, only to recant and restart the project a month later.
A 64-bit Firefox was important for Mozilla if only because of its push to retain users switching to Windows 10, which like previous editions of Microsoft's OS, comes in 32- or 64-bit versions.
Other improvements and changes in Firefox 43 include support for more intensive content-and-ad-tracker blocking using "lists" from San Francisco-based Disconnect, which also powers the "Private Browsing" mode introduced last month with Firefox 42.
Firefox 43, including the 64-bit version for Windows, can be downloaded from Mozilla's website.