Update: Google Wave Add-on for Firefox 0.0.3 has been released. Now Support for 3.7a1pre.
Google Wave Add-on for Firefox 0.0.1
It will notify you of new waves in your inbox.
It is still an experimental add-on, so you'll have to 'check' the box, before the web site will allow you to install in Firefox.
As it's an experimental add-on, for updates, you'll need to check the add-on page yourself for now.
After you install, you'll need to restart Firefox. After Firefox is started. Go to the Addon Preferences for and enter your username/password.
Note: It currently only works in Firefox 3.5.x.