Note that if you plan to freeze your Firefox versions because your Selenium tests break whenever Firefox updates, there is a better way that lets you keep an up-to-date Firefox. Mozilla has stated that they will no longer provide security patches for any but the most recent versions of Firefox. So running an old Firefox should not be a long-term solution for anything.

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

• Prevents you from data breaches and liability risks
• Upgrade at your own pace
• Works with modern Rubies

Read more Show archive.org snapshot

If you still wish to disable the auto-update in Firefox, a poster in this thread Show archive.org snapshot recommends bringing up about:config and setting the following to keys to false:

app.update.auto
app.update.autoUpdateEnabled