Requiring signing of web extensions is still ludicrous. It makes sense for Google to do it. It makes zero sense for Mozilla to do it.

Google wants to make money by developing a platform.

Mozilla (should) want to make technology that empowers people. Mozilla does not have to provide technical support. Mozilla isn't making money off of it. I think that the only reason they did it was because Google did it.

Should emacs or Autocad require signing of extensions? Why not? Because it would go directly against the grain of what those tools are for. And users should be smart enough to handle their own security.

I would like someone to justify Firefox extension signing, while also explaining why nobody cares if other software has signed extensions.

Because web browsers are the single largest and most common attack vector on every end-user device, and emacs and autocad aren't. I'm​ not in a position to say whether Mozilla's implementation of extension signing makes any sense, but it is practical to hold web browsers to higher standards.

I'm also not sure how extension signing makes Google any money?

Extension signing makes Google money because it supports their platform infrastructure. It supports things like Google Play. Microsoft also made money from Internet Explorer, even though they didn't charge money for it. They benefited from controlling the platform.

Mozilla has no such incentives. And they also don't have any larger strategy that Firefox supports.