When I create a new event or task, I have the option of including an audible reminder alert. By default, as I understand, this is the same default system sound that is played for new email. I had the event reminder sound customized until the most recent Thunderbird update, which changed it back to the default. I changed the sound that is played for new email back to what I had before, but that only affected the new email sound, and not the event reminder sound.

On an unrelated issue, I'd like to have task list only show tasks that repeat once, which I think is how it worked before the most recent update.