You can try:

• Bookmarks -> organize bookmarks -> organize -> undo
• Bookmarks -> organize bookmarks -> import/backup -> restore -> latest date (export current bookmarks to HTML first)

Many thanks for your reply, Gryllida.. An Apple tech helped me retrieve the bookmark folder by using Time Machine and returning Firefox to where it was an hour before the folder disappeared. What I'm concerned about now is that this will happen again.. Is Firefox unstable? What causes a bookmark folder to just vanish?

How do I delete a bookmark? explains the ways you can delete a bookmark.

Firefox's native behavior does not include bookmarks vanishing without a reason. Of course bugs exist and if you don't think that you did anything described at the article by any chance, you can try to find a pattern in your action and probably realise what steps to reproduce are.

However bugs are not usually as frequent in user experience as to cause data loss, so you probably would start with reading the article and trying to realise whether anything in the list can resemble your (probably accidental) previous actions.

It didn't work, the folder is still gone.

This can be a problem with the file places.sqlite that stores the bookmarks and the history.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox