You only need to backup what the article says, which I'll link here for reference because the link you posted is malformed:

https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data

The other profile folder you see under a Local folder is for temporary and cache files, and is kept in a separate directory precisely so you can backup the relevant data without including that.

You may want to backup the whole Thunderbird folder two levels above the profile folder, but definitely not the other part.

EDIT: Post added below to explicitly recommend, not just suggest, backing up the whole enclosing Thunderbird folder.

You only need to backup what the article says, which I'll link here for reference because the link you posted is malformed:

Sorry about the link - I noticed that as soon as I submitted the question but apparently editing posts is not a thing here?

The other profile folder you see under a Local folder is for temporary and cache files, and is kept in a separate directory precisely so you can backup the relevant data without including that. You may want to backup the whole Thunderbird folder two levels above the profile folder, but definitely not the other part.

Thank you - that helps a ton !

For reference, I've now figured out that my problem trying to migrate from one computer to another (that got me on this path in the first place) is that "way back when" I was short of disk space so I moved the Local POP3 folders to a different drive that had space. You can do that deep in the settings of an email account but if you forget to also back up that folder you can lose a lot. PEBKAC of course.

editing posts is not a thing here?

I do it all the time. You should be able to see a 3-vertical-dots button to the right of every post. If the post is yours, editing it should be one of the options available there.

editing posts is not a thing here?

I do it all the time. You should be able to see a 3-vertical-dots button to the right of every post. If the post is yours, editing it should be one of the options available there.

That was the first place I checked but all I get is the option to "Quote". I've attached a screenshot.

Maybe it's because I'm using a new account ?

[EDIT : well, I can edit this comment but still not my original post? ]

I'm going to somewhat correct myself and definitely recommend backing up not just the profile folder as the KB article says, but the whole Thunderbird folder, so that both the Profiles folder (where profiles are stored by default) AND the accompanying profiles.ini file (which Thunderbird uses to keep track of profiles) are included.

Backing up just the profile folder is enough to ensure the integrity of your Thunderbird data, but backing up the whole Thunderbird folder makes restoring to a different hard drive or moving to a different computer MUCH easier.

If your profile folder is located outside the Thunderbird folder for some reason, then backup that profile folder in addition to the whole Thunderbird folder.