Your browser doesn't NEED to natively support .pdf but it does and you've probably used that feature. I mean you could have it launch an external .pdf viewer, which is a FAR more complex piece of software than the little FTP protocol code they're disabling here.
Your browser doesn't NEED to support TABs. Your OS is already equipped with the ability to run multiple instances.
Your browser doesn't NEED to sandbox anything, you could just run each instance in it's own VM.
Your browser doesn't NEED to play video, it could launch an external viewer.