Binary browser plugins using the 1990s-era NPAPI ("Netscape Plugin API", the very name betraying its age) will soon be almost completely squeezed off the Web. Microsoft dropped NPAPI support in Internet Explorer 5.5, and its Edge browser in Windows 10 also drops support for ActiveX plugins. Google's Chrome started phasing out NPAPI support in April this year and dropped it entirely in September.
Now it's Firefox's turn. Netscape's open source descendent will be removing NPAPI plugin support by the end of 2016. Some variants of the browser, such as 64-bit Firefox for Windows, already lack this plugin support.
Mozilla's plans resemble Microsoft's and Google's in more than one way. There's one plugin that traditionally used NPAPI that's special: Flash. Chrome and Edge both embed and update their own versions of the Flash plugin, and even after 2016, Firefox will continue to support Flash. Though the scope and capabilities of HTML5 have continued to grow, Flash remains a significant part of the Web, especially for interactive content such as games. Many of these uses are declining, but support for Adobe's technology will still be a practical necessity in a general purpose browser at the end of 2016.
Again consistent with the other browsers, especially Chrome, Mozilla says that it will be working with Adobe to bring performance and stability improvements to its Flash support. Chrome's Flash does not use NPAPI, as Google was unhappy with both its performance characteristics and its security implications.
No such exceptions are on the cards for other plugins. Unity, which makes a cross-platform 3D engine that currently includes a plugin-based Web player, is going to drop support for its plugin in version 5.4, due March 2016. The company will continue to develop its WebGL version. This is currently less capable than the plugin, but Unity is working with browser developers to further increase browsers' capabilities to help fill these gaps.
Silverlight and Java applets will have to be discarded or replaced. The plugins responsible for these technologies already don't work in Edge or Chrome, and Internet Explorer is now essentially in stasis, receiving security updates but with new features and Web standards being exclusive to Edge. With Firefox dropping support, too, there won't be any mainstream, current, actively maintained browser that can handle these plugins.