I'd like to sync my Gmail work account to Thunderbird as a backup in case of a cyber attack. Is this a good thing to do in terms of security?

Any backup of important data is a good thing, compared to none at all. Security is very broad, volumes of books exist on the subject and they continue being written as technology keeps progressing. We couldn't possibly exhaust it here if we tried. If your concern is whether Thunderbird is secure to use for the purpose of backing up your Gmail work account emails, then you should know that, it is as secure as any other piece of software that is under active development and maintenance. It has known security holes (vulnerabilities) that get patched via regular updates for the definite serviceable life-cycle of a given product version, just like whichever software or app you routinely use to access your Gmail work account.

If I had to close my Gmail work account suddenly due to a cyber attack, how compromised would my synced emails in Thunderbird be and would I lose all the emails (or would I just stop getting new emails)?

That depends on how you choose to setup your Gmail account in Thunderbird, that is, which mail access protocol (POP or IMAP) you will choose to use. If, for example, you opt for IMAP, Thunderbird will by default keep a temporary local cache of your emails in the form of mbox files. One mbox file for each "folder" in your mailbox that it synchronizes for offline access on the computer on which Thunderbird is installed. If it faces problems with these mbox files for any reason, such as corruption/damage, it will discard them and create new ones and re-download all messages from the server as long as it can access that account via that server. On the event that it cannot access that account to re-download the messages and recreate the local cache, you will NOT be able to access them offline in Thunderbird! Mbox corruption and recreation does not happen a lot (we get relatively few reports here, of Thunderbird re-downloading messages from Gmail), but there is always that chance of it happening, and you do not want that to happen if the temporary locally cached messages were the only backup you had.

If you choose to use Thunderbird as a tool to backup any email account, I strongly suggest you also have another backup using a proper email archiving solution, such as Google's own Takeout facility and products such as Mailstore. Other email archiving/backup solutions do exist. Look into them as well.

Thanks Stans for your very thorough answer!

The problem I've found with Google Takeout is that when I select 2GB max file size for the exported Mbox files, all I get is one file that is 30GB+ rather than 15+ 2GB files. It seems to be a problem going back several years according to the Google support forums.

And then how do I open/handle such a large Mbox file?

Re Mailstore and other proper email archiving solutions - do you know of any others so that I can review them? And excuse my ignorance but what's the difference between backing up and archiving emails?

Thanks again!

The problem I've found with Google Takeout is that when I select 2GB max file size for the exported Mbox files, all I get is one file that is 30GB+ rather than 15+ 2GB files. It seems to be a problem going back several years according to the Google support forums. And then how do I open/handle such a large Mbox file?

ImportExportTools NG should work just fine, but you can import it into Thunderbird without using any add-on or import tool. I will link to the process as soon as I locate the thread.

Re Mailstore and other proper email archiving solutions - do you know of any others so that I can review them? And excuse my ignorance but what's the difference between backing up and archiving emails?

There is no difference really, it's more or less the same thing. You can archive email for backup purposes, or you can backup emails for archiving purposes! It's just word-play, the end result is the same.

Thanks Stans!!!

If you could find that thread re the best way to import a very large Mbox file into Thunderbird that would be amazing!

Some are featured here https://www.easeus.com/backup-utility/email-backup-software.html

A Google search for email backup solution will get you started.

I found the thread, but I see no reason why you should read through it all just to locate a paragraph of the solution, so I'll just write it here instead.

Rename your Google Takeout mbox file to remove the .mbox extension and to also give it a suitable name. Thunderbird does not use the .mbox or any extension on its mbox files. So, if the file is named All mail Including Spam and Trash.mbox, rename it to something like johndoe@gmail

Next, open your Thunderbird profile folder then go to the Mail > Local Folders directory.

Copy the renamed mbox file to this location.

Exit and relaunch Thunderbird. You will find the Takeout mail under the Local Folders account.

Thanks Stans!!! Re "You will find the Takeout mail under the Local Folders account." - so it doesn't how big the Takeout mboxfile is, Local Folders can handle it?

Actually, I think I found the answer to that in your reply to another one of my posts! Thanks Stan!