I use both Thunderbird and yahoo for e-mail and have been having a few problems with Yahoo for the last couple of weeks.My Yahoo e-mail layout has just been changed as well, not to my liking.Not sure if I am losing e-mails though, hard to check unless I was expecting a mail.
Thunderbird has been stable for me.
I have also had trouble with other Yahoo services with pages crashing or freezing. I will be installing windows 7 after xmas as I thought my vista was playing up.