if it is a Acer it might be "limpus light" that is the operating system
if so install a better operation system ( mint or ubuntu ) "limpus light" is the #1 worst distro in the world
but it also might be a different OS on the laptop so..
as to firefox
what did you do before it stopped working ?
install anything ?
open a terminal and start it from the command line
then post the error massage
also ? a question why this
with Right Click and selecting Run