I'm not gecko expert,
but it might make a sence to create such wave client as a standalone
XUL-application,
rather then as firefox add-on.
I dunno what advantages could give firefox browser environment.
(the question is asked in mozilla.dev.extensions group)
Current web-client in firefox(3.0.x and 3.5.x) lags like hell.
(
http://twitter.com/qmaxa/status/3329284211 )
I could not find out where is it's bottleneck.
Intuitively, i can suppose "too much javascript" in non-threading
environment.
Taking this in account,
writing pure-xul-based client may give some performance improvement,
simply by moving interface stuff into gecko,
and removing unnecessary web-browser-specific stuff.
And especially if to write XPCOM component for wave protocol.
But the latter is out of my competence.