email come to Thunderbird in bursts about every 15 minutes. That has not changed for decades. THere are push notifications where certain email are "pushed out to Thunderbird by the server on receipt. But that is not very reliable as it can only push around 5 folders and that is not always the inbox that is one of them. Which folders are "pushed" is determined by the server not Thunderbird and those that use priority type inboxes on webmail tend to have issues with push notifications..