Hello- Mozilla employee here, though not a Firefox developer. The good news is it's easy to change how we apply updates from completely silent to "download and choose" to "only check when I tell you". The bad news is, unlike many situations of security theater, we frequently ship security updates due to known existing and actively exploited security vulnerabilities. Staying up to date is the easiest and most practical way of keeping your browser secure. If you want the security updates but find the frequent feature changes jarring, we offer our Extended Support Releases here: http://www.mozilla.org/en-US/firefox/organizations/all.html. I hope you'll adjust your update settings to something that works well for you- we really are trying to keep users in control.