Mel,
Oh check your recycle bin - maybe it accidentally got clicked /deleted to there- if so you can select it and then click restore - it will go back where it came from.

If that doesn't do it-
do you know how to look on the hard drive to find Firefox /Mozilla ? {click on the My computer icon-to look at C: drive unless you have more then 1 drive C is usually the main one}

or redownload it