Why they need to do the things so complicated nowadays? Before there was the icon of a wheel, go to configuration, disable auto-updates, click on it. Now you need to go to the address above about:config and search for the next fields:
app.update.auto
app.update.enabled
app.update.silent
To the right, there’s a double arrow. By clicking on it you will change the configuration from true to false and vice versa. Solution found on StackOverflow (long time since the last time it appeared as a reference here!). Bonus: the procedure for old Firefox versions and the whole guide of the config for Firefox.
Bits and Dragons 