Firefox 52 is out today, and it's a landmark release for a couple of reasons.

The release is the final major version to support two legacy operating systems: Windows XP and Windows Vista. Future major versions of the browser will require at a minimum Windows 7. Firefox 52 is an Extended Support Release; it will receive security fixes (and only security fixes) for approximately one year. New features, however, will be restricted to the mainline version of Firefox. Microsoft no longer supports Windows XP at all, and Windows Vista drops out of extended support on April 11, 2017. Google dropped Windows XP and Windows Vista support in Chrome in April 2016.

As such, users of those operating systems will still have an actively patched browser for a little while longer, but their days are numbered.

Firefox 52 also gets rid of another bit of legacy: plugins using the old NPAPI plugin model, first introduced by Netscape back in the 1990s, are no longer supported, with just one exception: Flash. Other plugins, including Java, Silverlight, and Acrobat, are no longer supported. Chrome removed NPAPI support in September 2015, and Internet Explorer dropped it years ago; Microsoft's Edge browser includes neither NPAPI nor ActiveX plugin support.

This change means that there is now no actively maintained, supported, modern browser that supports the use of the Java or Silverlight plugins.

All that are left are legacy browsers. Internet Explorer 11, running on Windows 7, Windows 8.1, and Windows 10, continues to support the ActiveX versions of Java and Silverlight, but this browser is now in strict maintenance mode, with Microsoft only providing essential security fixes. Internet Explorer 9 is supported in Windows Vista until April and on Windows Server 2008 until January 2020. Internet Explorer 10 is supported on Windows Server 2012 until October 2023.

Update: The ESR builds of Firefox 52 (though not the main version) will also have NPAPI enabled, offering another option that will soon have legacy status.

But just as one era of Web extensibility ends, another begins.

In 2015, development of WebAssembly ("Wasm" for short) was started by Mozilla, Google, Microsoft, and Apple. WebAssembly is intended to be an efficient bytecode that can be run effectively by JavaScript virtual machines. Java, Flash, and Silverlight all provided virtual machines of their own, but they did so outside of the browser's standard sandbox. Many of the security flaws that those platforms suffered were due to their sandboxing being weaker than the one the browser provided. WebAssembly avoids this problem entirely. Wasm's binary, bytecode format should make it faster to download and execute than predecessor technologies like asm.js.

Firefox 52 includes support for the first iteration of Wasm. C and C++ programs can be compiled to Wasm using Emscripten, the same tool that is used for asm.js, and those programs can then be run in the browser. Mozilla's browser may be first, but it won't be alone for long; Chrome 57, currently in beta, is also due to include Wasm support, and Microsoft says that Edge support for Wasm is currently in development.