Windows 9x/Me
C:\Windows\Application Data\Mozilla\Firefox\Profiles\<Profile name>\
Windows 9x/Me, alternate
C:\Windows\Profiles\<Windows login/user name>\Application Data\Mozilla\Firefox\Profiles\<Profile name>\
Windows NT 4.x
C:\Winnt\Profiles\<Windows login/user name>\Application Data\Mozilla\Firefox\Profiles\<Profile name>\
Windows 2000/XP
C:\Documents and Settings\<Windows login/user name>\Application Data\Mozilla\Firefox\Profiles\<Profile name>\%APPDATA%\Mozilla\Firefox\Profiles\<Pr ofile name>\
Go to the appropriate directory and there should be a file called bookmarks.html
If it HAS become corrupted just create a new profile and migrate your settings into your new profile.