getfirebug.com has Firebug 1.6a1, the first release of our new branch, fbug/branches/firebug1.6. No new features yet, this is just our starting point and our way of checking patches that will be ported to Firebug 1.5. (BTW Firebug 1.5b9 should be out soon, and we expect it to be the last beta).
Firebug 1.6 will support Firefox 3.6 and 3.7, at least for now. We might later decide to focus Firebug 1.6 on features available only in Firefox 3.7. Many of the new features we plan for 1.6 require work in Firefox that probably will not be added to Firefox 3.6.
Users currently on Firebug 1.5 will not be automatically updated to Firebug 1.6.
Chromebug 1.6a1 is also available.