You can simply install an older version of Firefox over the current version. Please read this article for more details:

• Install an older version of Firefox

For security reasons, it is best not to roll back to Firefox 39. If you cannot run Firefox 40, you could consider a different variant of Firefox called the Extended Support Release (ESR). This is designed for companies that require a longer cycle time between feature changes, and currently is based on Firefox 38. It will get security updates over the next several months, but not get new features, so hopefully will stay compatible.

If you decide to switch, you can get an installer for Firefox 38.2.1esr from https://www.mozilla.org/firefox/organizations/all/ (Scroll down to your preferred language.)