Thunderbird email drafts saving to Mail 'draft' folder

For my money, the current version of Mail is the worst ever. I've had too many problems to list here but, today, a new one reared its ugly head. Replying to an email link, Thunderbird opened instead of my default Apple Mail. Since it was too much of a hassle to write down the email address and type into the Mail window I went ahead and composed my email. During the process, I was informed that Thunderbird was unable to save it to my draft folder. Was I worried? No! Later, however, when I went into Apple Mail two drafts of the email I sent in Thunderbird were in the Mail 'drafts' folder. Can anyone explain?