The functionality for moving individual folders in a Thunderbird email account to network locations

The functionality for moving individual folders in a Thunderbird email account to network locations is missing.

If you mean not included yes it is missing. Thunderbird stores the content of an account in a single location. Setting locations for individual folders is not and never has been an option.

Similarly, moving the entire mailbox to a network drive does not work correctly.

As you do not say what your experience has been I can not begin to comment

This is needed to free up resources on the local computer. In our case, the 500 GB SSD is occupied by a 350 GB mailbox.

You have 350Gb of mail? I have some 20 years worth in my profile and it is nothing like that much mail.

This is needed to free up resources on the local computer. In our case, the 500 GB SSD is occupied by a 350 GB mailbox.

It’s really cool that after 20 years you don’t even have to use email at all. But there are also work‑related needs — such as attaching large files to emails and storing them in archives for a very long time, 10..20..75 years. This includes a scenario involving the deletion of these emails from the basic mailbox on the server side, which may also be a commercial one with payment per gigabyte of stored information. I’m not talking about personal needs, but about work‑related ones. And there, you really do need to partially move the single email inbox and send emails to archived network locations.

350 GB is our reality. And this isn’t the maximum storage capacity we might need in the future. I think it’s x10. I’m not voicing a mere wish, but a real problem. Which I won’t be able to solve by tomorrow. Of course, I’ll swap the SSD for a 1 TB one at a huge price. But even that will be a poor solution to the problem of archival storage, which it’s preferable to store in structured "cold speeded" storage systems.