Yes, Outlook’s Ouath2 protocol will be supported.To me (and to my shame) the most important thing is: will Hotmail work with it? This OAuth2.0 thingy seems to break all. And while that even is available in a Thunderbird-ESR-Version running in El Capitan, it does not work at all there.
To speak PowerFox: PowerFox for Tiger works pretty well on Leopard PowerPC. Nice to have all the bug-fixes you promised. Can't wait to use the stable release! On Intel there are some quirks, which I may mention in the PowerFox Intel thread (I tested it in Snow Leopard, and Tiger Intel).