Just downloaded Firefox3 to try it out.

Good news:
Scribefire and FireFTP both still work. I use scribefire more than fireFTP but both as very useful extensions.
Tamper Data also works. Great for debugging.

Bad news:
Firebug doesn’t work. This is a horrendous killer for me. This extension is critical to my job.

In the 10 minutes I have played with it so far, it seems pretty good. I especially love the smaller shortcut bar, means it fits a lot more links than before.

UPDATE: Firebug does work, but it isn’t as good.  It used to output everything that was output to page, but I no longer see it anymore.  I used to be able to see what javascript built what frame etc, but can’t find that anymore.