All my shortcuts to the internet and all links in e mails have stopped working and I have no idea why. I have checked for the usual viruses but have found nothing. I use Mozilla.
It sounds as if your registry has lost the information telling your PC what your default browser is.
Try this:
Go to Tools > Options
With the 'Main' tab selected, check that there's a tick alongside 'Always check to see if Firefox is the default browser on startup. Then click 'Check now'.
If that doesn't work, it might be worth using System Restore, to set your PC's registry back to a date when your shortcuts and links worked.