Earlier quoted context omitted.
(Disclaimer: I'm working on http://www.webflow.com to try to solve this exact problem.) I've been thinking about this issue a lot over the last year, and it's really as hard as you state (and then some). We ended up having to create our own supersets of the DOM/CSSOM in order to keep track of individual elements/selectors as they are created/removed/changed. I would be very surprised (and pleased) if Mozilla was able…
I tried to enter my email address to get notification of your launch. Nothing happened when I clicked the button (using IE 8)
Edit: Should be fixed now.