Your Firefox bookmarks are stored in a database file (places.sqlite) in your personal settings folder (your Firefox profile folder).

The easiest way to locate your personal settings folder is from within Firefox:

Help > Troubleshooting Information > "Show Folder" button

Are you asking for a particular reason? Maybe: Restore bookmarks from backup or move them to another computer

See also:

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Windows XP: C:\Documents and Settings\yourusername\Application Data\Mozilla\Firefox\Profiles\xxxxxxxx.default\bookmarkbackups

where yourusername is the user name you logon to winxp with, and xxxxxxxx is random alphanumerics (there should only be one xxxxxxxx.default file in the location specified).

Windows 7: c:\Users\yourusername\AppData\Roaming\Mozilla\Firefox\Profiles\xxxxxxxx.default\bookmarkbackups

Can't help you with Win8 but imagine it's similar to win7. Good luck.