Firefox stopped using it on 2011-08-16. If you've seen Firefox using it recently when connecting through proxies, please let me know.
I'm sure you're right. It was on whatever version of Firefox was on my work laptop the week I learned Golang, since my first project with the language was an HTTPS/SSL proxy. I even griped about it on Twitter (it breaks Golang's crypto/tls), and (after we'd hacked support for it into our local tree) got a link to a patch. That would have been... meh... a year and a half two years ago.