deactivate Check for new messages every 10 minutes
I think leaving Check for new messages every 10 minutes by default is a bad choice. If the users wants this, he should activate it.
It adds unnecessary load to Tor if the user left his desktop for a while or simply forgot to close Thunderbird. Checking every 10 minutes lets the mail provider guess how long a Tor session lasted. Its better to pull mails only on demand and cease any unnecessary transmissions afterwards.