This hardly applies to Firefox, which is a browser. It would apply to Thunderbird or SeaMonkey.

I was in the Thunderbird help location and presumed any answers would apply to Thunderbird! The sent box has been in position ever since I started using Thunderbird some ten years ago. Why has it suddenly vanished and how can it be retrieved?

http://kb.mozillazine.org/Empty_folders There is advice about missing folders on this page as well as empty ones.

moving this to the Thunderbird queue

I am unable to find where the profile your mention is situated. I have been able to find the sent box, complete with its contents that are all up to date - so it is still working somewhere. 'Once found how do I get it to stay in its previous position under inbox/outbox/deleted' - incidentally drafts has gone as well - no idea where that is!

Please coonfirm you have this setting: View > Folders > All or Menu Icon > Folders > All

Yes, going in to "view" - "folders" - the "all" button is clicked. I have tried unclicking this and then re-instating it, but no joy.

Is this a Pop mail account using it's own set of folders? Is it a Pop mail account set up to use Global Inbox (Local folders)?

If IMAP mail account, then check to see if you need to subscribe to see those folders.

Thank you for taking time to respond to my question. Somehow the sent box has reappeared along with 7 other inboxes and the drafts folder! I really dont know what I did, but I had been right clicking on my email address at the top. I had actually tried this before - without success. Thanking you for your help.