See this: [https://support.mozilla.com/en-US/kb/Lost+Bookmarks]

First, do not close or open Firefox. That may erase your backups. In Windows Explorer or My Computer, navigate to your bookmarkbackups folder. Usually it's here: C:\Documents and Settings\''windowsusername''\Application Data\Mozilla\Firefox\Profiles\''profilename''\bookmarkbackups Copy anything that looks possibly useful. However, you might only have very recent backups here. If you are in the habit of backing up your computer to disk, tape, DVD, etc., then check your backups for this folder. If not, consider starting. [https://support.mozilla.com/en-US/kb/Backing+up+your+information Backing up your information].