This article explains how to do this: Install an older version of Firefox. However, if at all possible you should avoid this, as older versions have known security issues. For this reason, you should encourage your client to upgrade to a newer version of firefox as well.

Unless you are paid by the hour for solving problems in a 15-month old version of Firefox, I think it would be a better business strategy to only support the current versions of Firefox and Firefox ESR, and the immediately previous versions...

For security reasons, it's not a good idea to use Firefox 27 for your general browsing. Maybe you can use VirtualBox (https://www.virtualbox.org/wiki/Linux_Downloads) or similar container for your old Firefox to avoid disrupting your regular Firefox.

Note that on Linux it is easy to install and run multiple Firefox versions. You only need to download this Firefox version and extract the content of the tar.b2 archive to a folder. You need to create a new profile and a shortcut to make sure to start Firefox with this profile.

• https://developer.mozilla.org/Mozilla/Multiple_Firefox_Profiles

• http://download.cdn.mozilla.net/pub/mozilla.org/firefox/releases/27.0.1/
• http://kb.mozillazine.org/Installing_Firefox#Linux