You can skip 39 and go straight to Firefox 40. You can get the installer here (scroll down to your preferred language):

https://www.mozilla.org//firefox/all/

Let me guess... that wasn't what you meant. For your security, you should update, but I gather there is a particular reason you prefer Firefox 38 over later versions. What's the reason, maybe we can help with a solution or workaround?

If you want to stay with a Firefox 38 version then you can consider to switch to the Firefox 38.2.0 ESR build. That way you will receive security updates.

Firefox 38.2.0/ ESR:

• https://www.mozilla.org/en-US/firefox/organizations/all/

• https://www.mozilla.org/en-US/firefox/38.2.0/releasenotes/
• https://www.mozilla.org/en-US/firefox/organizations/system-requirements/