A Microsoft file hosting and synchronization service.
Since I used the Chrome web browser straight through (when my OneDrive folders/files failed to display (despite multiple successful logout and re-login), as well as when it eventually succeeded the next morning (again successfully logging in) without any other action), and WITHOUT resort to Edge or any other web browser, that implies the root cause is some other ill-defined transient problem, instead of a browser-dependent problem that might be fixed by attempting access with Edge before again attempting FireFox.
It seems we lack sufficient comparative data to speculate as to what else -- beyond <mysterious transient system problem at the Outlook server end> -- might apply.
For those whose OneDrive files have disappeared but did NOT re-appear within a day or so, it will require more information to track down, and may require Microsoft intervention.