I have been using Thunderbird as my email client for many years. I very often get a backlog of emails in my inboxes and so I delete those messages that I think are not important. When I delete them they go into the deleted folder for my attention at a later date. These messages have always stayed in the deleted folder until I permanently manualy delete them.
Twice recently all but about a weeks messages vanished from this folder. Only the deleted folder is affected and the emails do not appear on the VM server either.
I have only used about 3% of the available resources and I can not find why this is happening.
DO NOT USE the Trash or Deleted Folders for storage.
This is nothing to do with Thunderbird. I can't find the reference quickly, but Virgin auto-clears those folders on a regular basis (and the same is true of most email providers). Use the archive function (see webmail) or make your own folders for things you want to keep.
Personally, I double-delete things to remind myself that the trash folder is not a storage area, mine is always empty.