I then gave it a shot in Firefox nightly and it seemed to work really well. This brings me joy.
I don't like to edit forms/documents on the web; it's a constant struggle against my muscle memory. Vim.js could be the start of something that could change that. Well done!
I'll also note that in the past, I've purposefully not upgraded due to serious bugs in the present version of Chrome. (There was a great one a while ago where entire websites — notably Wikipedia — were rendering in the wrong font, for example.)
> a browser that silently auto-updates
While I'll grant that this is true for the majority of users:
1. It requires a user to restart their browser. For me, this usually means the next weekend or Chrome/OS X crash, since during the week I'm usually busy with things other than restarting the browser…
2. Not all OS's auto-update, though this is a very small minority.
I was merely providing feedback / datapoint.