1) Open the terminal and write the following lines:
"sudo apt-get update"
"sudo apt-get upgrade"
This will update also other packages, if you want to skip everything except Firefox write only the first one.
2) Write these lines:
"sudo add-apt-repository ppa:ubuntu-mozilla-security/ppa" "sudo apt-get update" "sudo apt-get install firefox"
Answer the terminal with Y to complete the passages.
Et voila, you have updated Firefox ;)