It's a little strange to me that when I add a new account in Thunderbird, it immediately starts downloading all the emails in it, as highlighted in this thread. I'm a completely new user and this caught me off guard as I had not expected it. Not only was space used up but several folders started appearing with hundreds of 'unread' messages which I had simply archived away.
Going into the synchronization/storage settings is too late, the 'clutter' has already been introduced in all folders. This is not a great onboarding/first-time-user-experience.
Idea is: