Don't, ever. There's no reason to trust Mozilla will keep this up forever. They're not even close to doing what uBO does. Blockers are best kept as a third-part

Don't, ever. There's no reason to trust Mozilla will keep this up forever. They're not even close to doing what uBO does. Blockers are best kept as a third-party solution with no conflict of interest. If too many people stop using uBO because the FF built in tools are "good enough", eventually we'll loose proper extension support and we will be at the mercy of browser vendors again.