Q: How does mozilla handle it when it's checking for new messages in the background and the user hits the get message button does it stop it's current check? or does it ignore the users request? or does it open another connection to the imap server? or does it queue the users request until it's done?
A: We ignore the second get message request, because nsImapMailFolder:UpdateMessages checks if we're currently running a url on the folder and if so, ignores the update request. If we didn't do that, we would queue it up after the background check, because we're not allowed to make two simultaneous imap connections to the same folder.
Note, this info only applies to imap. pop and nntp are different. (for example, news doesn't queue up, we'll just open a new connection)