Did you follow all the steps as mentioned in the mozillazine forums:
"File Types edit
(In Windows XP and Windows 2000)
1. Open Windows Explorer (or My Computer).
2. Go to Tools -> Folder Options -> File Types
3. Select Extension: "(NONE)" File Type: "URL:HyperText Transfer Protocol"
4. Click "Advanced". In the "Edit File Type" window, select "open" and click "Edit"
5. Uncheck "Use DDE" (the dialog should then hide the lower part).
6. Click OK for that dialog and the next one (afterwards, the "Use DDE" box is still checked but the "DDE Message" box will be cleared, as shown here)
7. Repeat for Extension: "(NONE)" File Type: "URL:HyperText Transfer Protocol with Privacy" (and any other protocols you want to fix)
8. Repeat for Extension: "(NONE)" File Type: "Firefox URL"
9. Repeat for Extension: "HTM" (or "HTML") File Type: "Firefox Document" "
Special emphasis on steps 3,7,8 &9.
Mine started opening in FF only, after ALL these Extensions were dealt with.("Use DDE" Unchecked)
Hope that Helps.If not I guess the experts have to come in!