Firefox users who prefer a 64-bit version of the web browser instead of a 32-bit version on Windows will be able to install and use a 64-bit stable channel version when Firefox 41 gets released.
Firefox 41, which will be released on September 22, 2015 if things go as planned, will be offered as a 32-bit and 64-bit version for the Windows platform.
All other channels of the browser, that is Beta, Developer and Nightly, are already provided as a 64-bit version, and that is also the case for the Linux and Mac OS X versions of the browser.
So why is the release, originally planned for Firefox 39 and then 40, delayed again? According to Mozilla, it is because of other improvements and changes launching in Firefox 41.
The organization mentioned sandboxing and NPAPI whitelisting in particular which it aims to deploy with Firefox 41.
One difference between 32-bit and 64-bit versions of Firefox for Windows is that the latter will restrict plugin access through the use of a whitelist.
Mozilla plans to put Flash on the whitelist, and maybe also Silverlight according to the official bug report on Bugzilla. Additional plugins are not mentioned which means -- subject to change as always -- that Java for instance won't work in 64-bit versions of Firefox for Windows because of that restriction.
It is unclear why Mozilla made the decision to integrate a whitelist in Firefox 64-bit for Windows, but the most likely explanation is security.
Firefox users may have two issues with this approach. First, not allowing certain plugins from being installed in Firefox will break some sites or applications. If Silverlight won't be supported for instance, it will break media streaming sites that rely on the technology. While many will move over to HTML5 eventually, it will take a while before the majority has completed that process.
Second, leaving Flash, one of the most dangerous plugins enabled in Firefox means that the browser is still open for plugin-based attacks.
Shumway, Mozilla's Flash replacement, is still not ready for primetime and it is unclear if it ever will be.
Some Firefox users will notice disruptions when they start to use the 64-bit version of Firefox for Windows due to this limitation. A workaround is not provided yet other than using the 32-bit version of the browser instead which is less restrictive.
Eventually though, NPAPI plugins will go away just like they did in Google Chrome for the most part.
Now You: What's your take on Mozilla's decision to limit plugin use in 64-bit versions of Firefox for Windows?
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