My contribution to the mozilla firefox project will be to review and enhance whatever is indicated by this metric:
When I change tabs, how quickly do you show me the contents of that tab?
This is important to me because of my particular (perhaps idiosyncratic) processor-intensive, browser-crashing, web-browsing style. Don't make me wait, ever. Never ever ever. I don't care what else may be happening. If you are a computer program with a user interface, your first priority needs to be interacting with me. Don't just do something; stand there. You are my guide; don't leave me stranded, even if to do something I asked you to do. If this means you need to (over-)specialize, then fine. Do it. Where 'it' is my most recent demand.
This is likely to be a hard problem and not localized to any particular part of the code.
An interesting first milestone will be to make a small suite of tests for this metric.