Google Wave is still available only to a subset of lucky users who've gotten invites for the service, but the buzz around it is huge. No wonder, then, that the first Google Wave related Firefox add-on is already available.
It's called Google Wave Notifier, and it has a simple function: it sits at the bottom right of Firefox and notifies you about new wavelets in your inbox.
Of course, to be able to use it, you need to supply a Google Wave username and password. The author warns that the plugin has been tested and works with Firefox 3.x.x but not version 3.7a1pre.
I regularly use a similar add-on (Gmail Manager) that notifies me about new messages in my Gmail accounts; Google Wave Notifier should be just as useful. You can find it here.