Thunderbird comes with 2 different mail protocols which are POP3 and IMAP.
Users must choose between this two whenever creating an account.
The difference between these 2 protocols are:-
IMAP : The emails are stored in the server. Whenever a change is done to Thunderbird,
the server will also be applied with the same change and any changes done on
the server will also be applied to Thunderbird.
POP3 : New emails are stored in the server, which will then be downloaded by
Thunderbird to store on user's computer. Once downloaded is completed,
the copy from the server will be deleted.
In deciding which protocol to be chosen, it depends on the type of usage that users desire.
Will the account be accessed from more than 1 device?
Do you have limited storage of your mailbox?
Most of the time, mail providers have given enough space for users to store everything on the server.
But if the user plan on storing more than 15 GB , choose POP3.