As long as the Test Pilot team is considering some pretty radical UI experiments, I thought I would suggest another experiment.
Location Bar Enhancer is a Firefox addon that changes the URL bar into an interactive "bread crumb" style navigator for the current web domain. To navigate up to a certain directory, even the website root, you just click on that subdirectory (or the domain name itself). This make traveling upward in a website's directory hierarchy more like moving higher in the local file tree of a desktop file manager, which I find quite useful.
Admittedly, this is a browser addon that's been around a while, so it's not a completely new experimental idea. However, the addon's audience has been relatively small, I think perhaps because their implementation allows too many optional preferences for such simple functionality, and as of this writing, it hasn't been updated in a while. I think the Mozilla team would do better by this interesting design concept, and I think something like Location Bar Enhancer has at least as good a chance of being accepted by main stream Firefox users as TabCenter (which I'm also a big fan of).