I don't have any suggestions, only to add that I'm a Yahoo/Firefox user too, and every now and then the hard drive will start spinning like crazy. Taking a look at the performance stats, it's Firefox that seems to be the culprit.

A new Yahoo mail problem that recently started (after years of using the same setup) is that occasionally an incoming e-mail doesn't download from Yahoo to our Thunderbird e-mail. Never had that problem when using Netscape, but when Netscape ended it was continued with Firefox.