Open 2 instances of your file manager. In one, open ~/.mozilla/firefox and in the other ~/.mozilla-old/firefox. You will see some folders. Open them one at a time to see what is there. When you see a folder named bookmarkbackups, right click the one in ~/.mozilla-old/firefox/<name-of-folder>/ and choose Copy. Then in ~/.mozilla/firefox/<name-of-folder>/ right click and choose Paste. When asked, choose to overwrite.