If you haven't already heard, Adobe is going to stop Flash development for mobile browsers, which given the rising importance of mobile means they're effectively giving up on Flash on the Web altogether. Now there are strong noises that Microsoft is going to stop Silverlight development. This comes on top of Windows 8 Metro mode not supporting plugins (which of course follows Apple's early lead in iOS). This is huge! It means that browser plugins are going to go away. Oracle hasn't said anything yet, but there's no way browser vendors are going to carry NPAPI support forever, or add any new NPAPI features now, just to run Java and some other relatively little-used plugins. (This is not official Mozilla policy, mind you, just my prediction.)
What does this mean to Mozilla? It means we need to stop investing in new features for better integration with NPAPI plugins. Our investment in NPAPI support should focus solely on improving the user experience for consumers of existing plugin content, but limited by the knowledge that the consumption of such content --- and production of new content --- will decline dramatically over time. There's still probably quite a few things we should do there. Eventually we'll be able to disable plugins and rip out a lot of code.
What does this mean for the Web? This is a big victory for open Web standards. Two powerful proprietary competitors have been defeated. Sooner or later we'll be able to stop taking plugins into account when we design new features (e.g. our full-screen support has to consider how plugins work in a full-screen document).