The steps below guide you through setting the configuration files required for Firefox to not only turn off updates, but deny the setting entirely.
Also note, this is on a per computer basis. This process will need to be replicated on each computer/server as necessary or desire.
Note: The install directories used here is default for a Windows 64-bit operating system. You install path may different.
Create file local-settings.js in C:\Program Files (x86)\Mozilla Firefox\defaults\pref
Copy the following and save in file
Create file mozilla.cfg in C:\Program Files (x86)\Mozilla Firefox
Test and confirm access
You can confirm this by going to the Preferences and Update tab and find that the update options are greyed out. Alternatively, browse to about:config and the above options will be in Italics.