* Think very hard before adding things to make sure that they actually need to be in the core browser. Looking at you, FF Hello.
* Don't baby the user. Everything should be configurable. And existing options should not be changed on upgrades. And general solutions are (always) better than centralized services. Looking at you, Firefox "You cannot install unsigned extensions, period".
* Don't introduce "features" that hurt the end user. Looking at you, Pocket "you now are bound by the terms of Pocket's user agreement when you run FF".
* Don't try to do cat-and-mouse games. Looking at you, Firefox "we must try to prevent malware on the user's computer from taking over FF".
* Don't assume that everyone has the newest and best computer. Looking at you, Firefox "electrolysis tripling memory usage is fine, right?".
* Don't couple security updates with anything else. Looking at you, Firefox "you must downgrade to australis or else your browser will be unsecure aah!".