I have accumulated many folders containing past emails in Yahoo Mail. I would be loathe to change to Thunderbird for that reason as it would mean swapping back to Yahoo Mail to find past emails.
I believe you can't use Thunderbird for Yahoo Mail any more than you can use Outlook Express for it. Yahoo Mail is not a POP mail account; it's a webmail type of email account.
Is there some way I can switch off the the Yahoo Mail processing that is slowing down my computer so drastically? Or is it something else that has caused this problem to emerge?
Well, do you need to keep Yahoo Mail open all the time? If not, then just close it.
By the way, tell us more about your computer (processor, RAM, etc.). Since you're using Windows 98, what version of Firefox are you using?