Welcome to PressF1 -
Sounds a bit like your profile in firefox is damaged.
Go to This Site / Forum and follow the instructions to replace your profile.
First open "My Computer" select Tools>Folder Options>View> Select " show Hidden Files & Folders > Click OK
Firefox is located in C:\Documents and Settings\"user name"\Application Data\Mozilla\Firefox\Profiles
Navigate to the folder containing Firefox as described above and there you will find bookmarks.html - copy this to a safe location, you may want to copy the Bookmarks backup as well.
One other thing you can try is Here Create a new profile then dump the other one. (save bookmarks first) But usually this is only a temp fix as the new profile sometimes can also get damaged right away.