If you want to store messages locally so that they are independent of any servers or Internet connections then you should look at:

1. Using POP, which by default removes messages from the server and saves them on your computer's disk drives, or,

2. Moving messages you want to keep into Thunderbird's Local Folders account, which again, saves them on your computer's disk drive.

In either case, you are in a position where the copy on your computer is potentially the only copy and therefore you need to take steps to back it up. Backing up your entire Thunderbird profile is the simplest solution, but I'd invite you to consider extending this to backing up your computer rather than just your email profile.

I think you may have your priorities the wrong way round. Your email provider probably has professionally-run servers with backup and redundancy and it's unlikely they would lose your data, whereas a single point of failure (motherboard, or worse, disk drive) could render your computer (and therefore your email collection) inaccessible.

But a computer will in all likelihood contain much more than just email, and a lot of this data may exist only on your computer. So back it ALL up!

Zenos said

If you want to store messages locally so that they are independent of any servers or Internet connections then you should look at: 1. Using POP, which by default removes messages from the server and saves them on your computer's disk drives, or, 2. Moving messages you want to keep into Thunderbird's Local Folders account, which again, saves them on your computer's disk drive. In either case, you are in a position where the copy on your computer is potentially the only copy and therefore you need to take steps to back it up. Backing up your entire Thunderbird profile is the simplest solution, but I'd invite you to consider extending this to backing up your computer rather than just your email profile. I think you may have your priorities the wrong way round. Your email provider probably has professionally-run servers with backup and redundancy and it's unlikely they would lose your data, whereas a single point of failure (motherboard, or worse, disk drive) could render your computer (and therefore your email collection) inaccessible. But a computer will in all likelihood contain much more than just email, and a lot of this data may exist only on your computer. So back it ALL up!

Thank you for the informative answer. Now that I think about it, what I want is to have a backup copy of all my email messages, just as a backup - without removing them from the server.

If I back up my Thunderbird profile, and then, separately, suddenly all of my email messages get deleted from Gmail's server, would the Thunderbird profile backup allow me to restore all of the emails that were deleted from the server?

If not, is there a way to copy all of my email messages onto my hard drive to have them as a backup (without removing the email messages from the server)?

Backing up your profile is recommended, ideally onto a separate hard disk (or similar) so that if your computer fails you still have an independent backup to refer to. So, naturally, we'd advise you back up all your personal data, and not just the Thunderbird profile.

We have two types of email account; POP, which we looked at earlier, and the alternative, IMAP.

IMAP relies on the server to store all your messages. Thunderbird will keep local copies, mainly to assist searching. But the local copies are "synchronized" to the server, so if Thunderbird sees a change meaning that the local and remote copies are different, it will make whatever adjustments are needed to bring them back into line.

If you have written a new message, or created a new message by replying to or forwarding an existing message, Thunderbird will offer the new message to the server. However, if you just copy messages into the accounts' folders (for example, by replacing the profile) then Thunderbird will see a difference and will resolve it in favour of the server. Your newly-pasted messages would be removed.

For this reason, a back-up profile is useful, but can't heal all problems. In most cases, with IMAP, the risk of losing the messages held on the server is very small. Generally, if moving Thunderbird to a new computer, you don't need to bother with the contents of IMAP-connected accounts; once set up, they will just download everything from the server.

If you do find yourself in a position where the only copy of your messages is in a backed-up profile, then we'd look at moving the folders containing the lost messages into Thunderbird's Local Folders account, where they can sit without any reliance or reference to a server.

So this goes back to my previous comment; if you want to back up messages in an IMAP-connected account, I think your best approach is to move or copy them to the Local Folders account and then back up the profile. That way you'll have the messages stored locally, and an independent copy of your local store.

Another approach is to export precious messages to some external file, but personally I find that so many valuable features of email (e.g. searching, chaining of threads and conversations) are lost and this reduces the value of such an archive. If you do wish to export a large number of messages in this way, then I'd suggest you consider the ImportExportTools add-on.