Just trying it now.
The spell checker is working, sort of. It works, but I cannot add a new dictionary...I am stuck with the US dictionary at the moment. It tells you to use the Thunderbird versions, but that doesn't work either.
You now have a choice of Feed Readers. It checks for anti-phishing sites.
Extensions and themes are now called Add Ons.
Seems okay up to now. Though the dictionary seems a bit limited...see the attachment
Just make sure that you have the nightly tester tools extension, to make your extensions compatible.