There's no undo for bookmark changes,* but Firefox periodically makes backups of your bookmarks. You could restore one of those.

If you have made changes since the last backup, you may want to export your bookmarks to the HTML format -- before restoring that older backup -- so you have a record of those changes. You then could import from that file after your restore and rescue the ones you need. This can create a lot of duplicates, so it's not for everyone.

These articles have the steps for these various options:

• Restore bookmarks from backup or move them to another computer
• Export Firefox bookmarks to an HTML file to back up or transfer bookmarks
• Import Bookmarks from an HTML file

Can you get back to where you want to be?

Note that if you did a LOT of re-org since your last backup, you also could do it the other way around:

(1) Back up what you have now to a new JSON-format backup file;
(2) Restore the earlier backup;
(3) Clear everything except the missing folder;
(4) Export to an HTML file;
(5) Restore the backup you created in step #1; then finally
(6) Import the HTML file you created in step #4.

* According to the post below, there is some level of undo available in the Library dialog (Ctrl+Shift+b)

I backed up my book marks to a JSON file,but now Firefox wont recognise that JSON file,and now ive lost all the book marks i made in between posting here and receiving your reply.  :(

Sorry to hear that. Do you get a specific error message -- if so, what is it?

If you open a new tab and drag and drop your JSON file onto that tab, does it start off with something like this:

{"title":"","id":1,"dateAdded"

Make sure that the JSON backup has a .json (lowercase) file extension.
Windows shows the type as JSON in the Properties.

Note that if you accidentally deleted bookmarks that you can use "Organize > Undo" in the bookmarks manager (Bookmarks > Show All Bookmarks) to get them back.
The Organize button is the first of the three buttons next to the Back and Forward buttons on the toolbar in the Library (Bookmarks Manager).
That only works if you haven't closed Firefox.

i have a similar question. i deleted my bookmark either, but i have same bookmark in my chrome,how can i use that in mozilla?

See https://support.mozilla.org/kb/Importing+bookmarks+from+Google+Chrome

Yep ,spot on.It looks like the actual bookmarks are in the text,but how to get them out is another thing.

You can open the Bookmarks Manager in Google Chrome (icon with the three bars > Bookmarks > Bookmarks Manager) and select "Organize > Export Bookmarks to HTML File"