Use the "ImportExportTools NG" add-on to save your folders locally.
If you have the Outlook desktop app (part of Microsoft 365), use the wizard to save your data to a local .pst file. Thunderbird :
Click . This creates a copy of your mailbox in a PST file. Desktop Outlook App :
: Unofficial versions may be unstable, leading to permanent loss of the emails you are trying to back up.
: Only download new emails to save time and space.