I have Norton Internet security and have just had a message pop up saying 'High disk usage by Firefox'. What does this mean and is it anything to worry about? TIA.
Norton is known to pop-up the message you have had. woofgang's explanation is correct.
My guess is that, possibly, you have a number of Firefox Add-Ons. In which case, if the message keeps popping-up, you could try disabling a few to see if it makes a difference.