Thunderbird crashes when it's been open for a long time

[dazco]

If i have it open for any long period, maybe 10 minutes, maybe an hour, i never really thought about it. But if it's been open a while and i write a email, when i try and send it it will sit there saying it's copying it to the sent folder (i think) endlessly. It never sends or closes that message. So i close it and when i try and open it again it tells me thunderbird is running and asks me if i want to close it. I choose yes and after about 10 seconds it opens again. Does anyone know what causes this?

[DanRaisch]

What version of Thunderbird?
How is the account set up, as POP or IMAP? In Thunderbird the menu path is Tools->Account Settings->Server Settings->Server Type at the top right of the dialogue.
What is the target folder for storage of Sent messages? Find out using menu path Tools->Account Settings->Copies & Folders.