I think that you can use Sync to sync the MAC configuration to other devices. You should be able to enable sync in its settings. I don't know if you can backup this information, but it is stored in indexedDB (SQLite database in the storage folder) and is not easy to transfer or backup.

• https://addons.mozilla.org/firefox/addon/multi-account-containers/

Thanks for the information! I'm looking for it now. I copied a bunch of stuff from the Local and Roaming profile folders (carefully), but I can't find indexedDB.sqlite on my old machine's SDD at all (or indexed*.sqlite). I'm a "computer guy" as they used to call software developers decades ago. Now I feel dumber with age.  :-)

Man, I hate letting the computer win.

Like I wrote: it is not easy. IndexdDB is implemented via the storage folder in the profile folder, but there are no folder names in plain text, but folder names with a UUID that is linked to an extension via extensions.webextensions.uuids in prefs.js (about:config). You can access this storage via about:debugging (This Firefox -> MAC -> Inspect).

Can't you use that old device and possibly enable sync ?

You can also try to contact its creator for possible suggestions to see if he can offer an easier solution.

I am so sorry I forgot to let you know that I do sync Firefox across multiple machines. But it doesn't sync this. I tried to find out how to enable that before I posted. I guess it's to-do.

Awesome! Thanks very much. (I was confused about the meaning of storage in this context, but you got me going.) Thank you again cor-el!