Since upgrading to Thunderbird 78.6.1 my email accounts that are IMAP are performing very poorly. In particular, every time I select a message Thunderbird re-downloads the entire message from the server. It takes many minutes for Thunderbird to start as it seems to be redownloading all messages in every IMAP account every time. This makes the larger accounts with many messages unusable. It wasn't like this before the last upgrade. Is there some setting I need to change or is this a bug introduced in the latest Thunderbird? Thanks for any guidance.