Once Google Chrome was the first to announce that they won’t be supporting NPAPI anymore, they were also the first to provide a new architecture in order to rewrite your code to work on their browser. You can take a look on Native Messaging, which “can exchange messages with native applications using an API that is similar to the other message passing APIs”. The problem is that this approach only works on Chrome, is not something that you can adapt to other browsers.
A more useful approach is FireBreath, a browser plugin in a post NPAPI world. Check the words below from one buddy of the project:
“FireBreath 2 will allow you to write a plugin that works in NPAPI, ActiveX, or through Native Messaging; it’s getting close to ready to go into beta. It doesn’t have any kind of real drawing support, but would work for what you describe. The install process is a bit of a pain, but it works. The FireWyrm protocol that the native messaging component uses could be used with any connection that allows passing text data; it should be possible to make it work with js-ctypes on firefox or plausibly WEB-RTC or even CORS AJAX in some way. For now the only thing we needed to solve was Chrome, but we did it in a way that should be pretty portable to other technologies.”