Ask HN: Firefox extension for real-time notifications. Where do I start?

Hey, thought I'd ask the HN community because this seems like a question others may be interested in.

I'd like to build a Firefox extension for my website that pushes an alert to a user when one of their friends does something on my site.

I imagine the easiest (but less scalable) method would be to have the extension poll my site for updates, but that's going to hurt performance if this thing takes off.

Would it make sense to setup a jabber server for this? Say 1,000 people have this extension installed and are currently online, how resource intensive would this be (would I need a bunch of jabber servers to handle all these connections)?

It would be awesome if there was a real-time notification APIs that would allow me to do something like this without running the jabber network myself.. even if it cost money.

Thanks!