The perfect companion to Portable Thunderbird - Portable YPops!
For those not familar, YPops is an open source gateway to yahoo mail - it basically acts as a localhost POP server, allowing you to download your yahoo mail email into thunderbird, or whatever other pop3 client you are using. Very handy.
This forum from Mozillazine has more info on related systems for Hotmail etc etc, many of which could also be candidates for portablisation.
http://tinyurl.com/e3e2s