Earlier quoted context omitted.
Not sure if you're referring to something old, I've only been in Spain since 2012 but I'm having zero issues with the digital certificate for various government websites, from hacienda to local city governments websites. Never have I been forced to install Java either. I usually use Firefox on Linux and seems to work fine, at least for me.
It was just yesterday. To sign in you need @clave, but to actually sign the document, you need AutoFirma, that's the java-based tool.
That's strange, I've never needed @clave nor AutoFirm (and in fact, I've never even heard of AutoFirma before). I just use the digital certificate (for signing in and for signing documents) received from the government to sign everything directly in the browser, without any Java or anything else really.