you'll need to edit this file:
windows: <install>/win32/extensions/prism-runtime\@developer.mozilla.org/defaults/preferences/preferences.
js
mac: <install>/macos/prism/Prism.app/Contents/Resources/extensions/prism-runtime\@developer.mozilla.org/defaults/preferences/preferences.
js
linux: <install>/linux/extensions/prism-runtime\@developer.mozilla.org/defaults/preferences/preferences.
js
Find the follow line:
pref("app.update.enabled", true);
and set that to false from true. Then you'll need to restart the GUI window.