I know about the extensions, it's exactly how I was able to run that specific applet, but that's a high friction experience for the user. If the applet does not run it's very unlikely I will install an extension (or search for a Firefox alternative) unless I'm very motivated.

I'm thinking more like a line or two you can add to your pages hosting applets, and it would enable any modern browser to use it. Minimal friction for the webmaster, zero friction for users.