The system restore should have worked. I was surprised that it did not work for you, even taking it to 3 days before. You didn't had any other restore point earlier than that?
Which was the installation disk that you used, when you had installed your OS? I think you need to use that disk when sfc asks for the CD.
Looking at the present situation that your system is in, with so much problems, last measure would be perform a clean reinstall. If you don't want to do that now, then you should try the Windows Repair option. In case you don't know how to carry that out, you can see here:
http://www.michaelstevenstech.com/XPrepairinstall.htm
From the above site:
A Repair Install will replace the system files with the files on the XP CD used for the Repair Install. It will leave your applications and settings intact, but Windows updates will need to be reapplied.
Windows repair might be a good option for you at this time. You should give it a try. You should also backup the data on your disk, in case something goes wrong.
And don't be afraid to carry out any actions on windows. If in doubt, read about it from the internet. Many help sites are available. Make a backup of your disk, so that if anything goes wrong, at least your data is safe. And that will give you confidence too, to carry out any operations on windows.
And what about Firefox... what version are you using? Did you try uninstalling Firefox, and reinstalling it again?