If it truly is as you say, your profile might be corrupted. You'll have to create a new profile, and restore your bookmarks from the old one.
1. Go to the Run prompt in your Start Menu, and type firefox -profilemanager, then press Enter. This'll open up the profile manager.
2. Create a new profile, name it whatever you want, and CLOSE the browser once the profile is created. (You might want to UNcheck "Do not ask at startup" for the next few steps, or leave it unchecked permanently like me -- I'll tell you why later.)
3. Go to the Run prompt again, and type %appdata%, and press Enter. This'll take you to your Application Data folder.
4. Inside AppData, locate the Mozilla\Firefox\Profiles folder.
5. You'll see two folders there; both will have a random string at the start; one will probably end with "default" and the other with the name you just gave your new profile. Enter "default".
6. Inside "default", locate the bookmarks.html file, and copy it to the OTHER folder (the one with your new profile), replacing the new file.
7. Run firefox -profilemanager again, and pick your shiny new profile. It should have the bookmarks from your old profile, and you can restart from there.
I myself keep the profile manager always active because my main profile is somewhat clogged with quite a few things, so I always keep a light profile with only a few extensions (the Google toolbar and two or three others) for quickly checking stuff. For all my main navigation I use the main profile. Hope this helps.