I have a number of filters set up and usually Thunderbird takes x number of emails and sorts them into various folders, Work, Personal etc. With the latest install of the software (on a new PC) I've noticed it will download x number of emails and only flag up a few of them by highlighting the folder in blue and the number of emails. Eg. one folder will get (1) at the end of it with one new email. Another will receive the filtered email but not show any highlighting. It's there if I look but there's no notification. This is really dangerous as I am regularly missing work emails as they silently build in the folders with no notifications!