[Thorrez]

Browsers can only protect against malicious websites and malicious extensions. They can't protect against malware. Even without any cert problems, malware on your machine can modify the browser executable/process to insert whatever code it wants.

With this reduced threat model, it's easy to simply keep existing pre-installed extensions available, and disable updates. Your only problem is if a pre-installed extension is malicious or has a vulnerability, it will remain.