The article explains this. We built the infrastructure, and we used it for out-of-process plugins on desktop, and for Firefox OS, but we went in search of lower

The article explains this. We built the infrastructure, and we used it for out-of-process plugins on desktop, and for Firefox OS, but we went in search of lower-hanging fruit on desktop. There was also (IIRC) some worry about breaking extension compatibility, which would make it a nonstarter. I think there are some planned mitigations for that now, and we've fixed a lot of the easy wins for responsiveness, so we're pursuing this again.