I noticed Firefox getting janky not long after startup and discovered that TestPilot's remote-experiment-loader.js was monopolizing the main thread for about 600ms. From looking at the profile alone, it looks like the time is being spent opening a Zip archive and checking file mtimes.
TestPilot version 1.2.2
We should look into moving some of the I/O off the main thread. I'm told it's not currently possible to open archives off the main thread.
btw, should pass ".js$" into findEntries in http://email@example.com/modules/jar-code-store.js#53
Fixed by removing test pilot 1. Any new versions we integrate should not have this kind of problem.