You're switching between favourite mailboxes, not accounts as such. Outlook does the same.I don't need to know that it's an inbox. I just need to know which email address I'm switching between.
It's an option in the account settings (sync all messages regardless of age, or sync the most recent NN days). The problem is they default to all, whereas the Mail app defaults to the last 14 days I believe (and can be changed to all).I'm still really not happy with Thunderbird though. Why should I have to have all my emails downloaded onto my computer? Isn't the purpose of IMAP so that you don't have to deal with that garbage?
I just tested the latest Thunderbird with an outlook.com account and they still don't support Exchange natively. That would be OK for some if they helped the user connect to online contacts and calendar with standard protocols. How hard would it be to build-in settings for those outlook.com services? I can't be bothered looking up the server names etc, don't need to do it for Mail/People/Calendar or for emClient/Outlook.
So the account setup procedure is still quite poor in Thunderbird (haven't tried gmail though).