Added an indicator of duration to the Firefox extension visualizations

As discussed on Friday, added an indicator of how long each request took to process. I did it in the easiest possible way. It would probably be better to adjust all visualizations to have relative scale, so that for example a 1ms request would look small next to a 100ms request, but really doing that right would take a disproportionate amount of time.

Here's what it looks like; hovering over the visualization makes the indicator opaque:

![temp](https://f.cloud.github.com/assets/203177/665375/3615012e-d79c-11e2-93a7-6453e85a8038.png)