For me Firefox 18.0.2 has been the best Firefox until now in terms of performance, especially with flash videos with several tabs open, so I'm glad they are improving performance even more. Start-up performance, even though it's relative if it's that important to all people, has been one of Firefox's main drawbacks compared to other browsers, so it's always good to know when they make improvements on that.
Also good to know about the built-in PDF viewer. I had switched from Adobe Reader to Foxit Reader, and to maximize performance and security within the browser I had even disabled the plug-in, so that Firefox would request to open the main Foxit application to open the file, it will be interesting to see how their HTML5 implementation works, both from a performance and a security point of view.