According to the article, you can type:

    resize to 320 480

There's a bug in bugzilla for that. Also, I think you'll find that the whole issue of "common phone screen resolutions" is actually painfully complicated. The iPhones, for example, report their resolution as half the actual resolution (in each dimension).

You can resize the view port to whatever size you want, even in Firefox 15. The resize handle is a little dark in 15, though.