I am hopeful, as this could finally entice the Firefox developers to create better features out of the box, unlike previously when the browser relied mainly on extensions even for core features. It didn't help that the main developers (especially Asa Dotzler) were dismissive of criticism and didn't improve the product significantly for a long time.
I have been using the product since Phoenix (probably v0.4, don't remember exactly) in and out, but it was never possible to make it my main browser because it took hours to find the right extensions, and after that they stopped working with each new version. The lack of features out of the box was the killer.
Many here will remember that Firefox mostly achieved its success because the competition was really bad in the beginning, specifically IE 6, and Opera cost money at the time. When Opera became free, it was too little too late, and by the time it solved some basic rendering problems and started gaining popularity, Chrome came out and was essentially better than both browsers from the start.
I really hope the planned versions of Firefox provide a good alternative to Chrome and its clones, I really hope Webkit doesn't become a monopoly.