And how do the two interact? Does Firefox's built-in tracking protection block content before uBlock Origin even has a chance to see it, and if so then is using the two in combination faster than just using uBlock Origin alone?
Extensions operate on requests before the platform-level channel classifier.
In theory, the platform-level classifier should be faster than the webRequest API, but we've not measured it specifically.
Running both uBO and and Tracking Protection can give some extra coverage; I've noticed sometimes the Tracking Protection shield pops up even though uBO (and Privacy Badger) are running.