Firefox 21 was released this past week, so it's possible you're seeing something related to an update. But... I've never heard of those files before. If you right-click > Properties on each of those shortcuts, what are the Target paths?

I believe this occurs if the install process is interrupted. Commonly windows update or a loss of power is the culprit. May of been an installation you did not initiate that mozilla maintenance service was running.

I would run Clamwin and then if all is well. Delete the 2 icons and manually update firefox if necessary.

When I click properties on the shortcut tab there is target listed on the left but in the box it reads C Documents and Settings.

I wondered why these appeared on the desktop spontaneously on Friday 11:00 ctd. Is it OK to simpley ignore it or should I delete?

Thanks!

How do you manually update? Do I need to update?

It does appear odd that 2 shortcuts magically appeared on my desktop.

The value of target probably is longer than that. You might need to click and drag across it to the right to see/select the whole thing.

You can update Firefox using

Help > About Firefox > "Check for Updates" button

Firefox updates usually contain bug fixes and remove security vulnerabilities, so while you don't always need to update the week an update is available, prompt updating is a good idea.