Ain't it possible that every message I send or forward will just be replaced to the outbox and will be send by Thunderbird in the background ? I really hate it that every message sends itself away, running on top of all other windows, and it makes me wait till it has been sent from reading my other messages...

Personally I don’t find necessary to see the whole progress of how a mail is sent step-by-step, I just want to send the e-mail. Can’t understand why they are showing me all these but anyway

An alternative way to remove the All Mail folder would be to login into Gmail webmail using a browser, left click on the gear icon in the upper right corner and select Settings, select the Labels tab, find All Mail, click on Hide and uncheck "Show in IMAP". Logout and delete "All Mail." and "All Mail.msf" in the Gmail accounts local directory in the Thunderbird profile.

Do NOT try to delete the All Mail folder by deleting its contents. That will delete all of the messages for the account when Thunderbird syncs the folder.

The "All Mail" folder in a Gmail IMAP account has a copy of all messages for that account, doubling the number of messages downloaded for offline folders. Thunderbird tries to download only one copy of a message from a Gmail IMAP account and have the folders point to that copy. However, that doesn't help if the message was created using Thunderbird. [1] If you decide to keep offline folders enabled and have a Gmail IMAP account, uncheck "All Mail" in Tools -> Account Settings -> Account Name -> Synchronization & Storage -> Advanced. As a precaution right click on the Gmail account name in the folder pane, select subscribe in the context menu, expand the folder listing and verify the All Mail folder is not subscribed. Disabling it from being synced should have unsubscribed it. Exit Thunderbird and delete "All Mail." and "All Mail.msf" in the accounts local directory.

However, I don't want or need any email from the account other than when I check it manually.

From here, you can configure the FIRST SYNCHRONIZATION OPTION (Important! If you mess with the lower options you might delete copies on the mail server).

Compacting folders does nothing for me -- I don't know why. I compacted them today for the first time in about a year, and the folder size remained unchanged. I don't generally delete emails, so that's likely why, but that doesn't mean I need to keep local copies of 12.5GB of emails.