Hi - well playing around with the DTrace Mozilla load probes for URI's and created a timing script to give us a breakdown on average and total load times for a URI based on it's Scheme, Content Type, Username, File Extension, Host and Site. I've uploaded a sample run and the script.
This is just looking at URI's and is only looking at network activity, but it's pretty cool to see why my page loads sometimes suck from the network side of things 
I'll need to start adding more probes to other parts of the stack to see how much time is being spent in DNS lookups, parsing, rendering, painting and so on, but it's a start 
LoadURI LoadImage
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Network --> DNS Request
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Parser
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Content DOM
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Layout
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Painting