Mail.app opens by itself while I am using Thunderbird

Hi, Eric Thanks for quick response. I prefer Thunderbird. Since I fooled around with the mail.app yesterday, I have been having firefox problems, millions of documents downloaded in seconds. There is some kind of tie to the preview.app also. I may get back to you with more questions. This whole thing is driving me nuts. I'm trying to write a novel here, I don't want to join the cloud or sync or do anything but finish my book. (Venting is good for me, apologies to you for taking up time.) I have an older laptop with Snow Leopard, never had this kind of trouble. Progress for Apple is not progress for me!