Just thought I'd share that (if you're a Firefox OS hacker, you want to read this).
We recently landed a very basic timeline in the Firefox DevTools (enable it in the devtools options. Firefox and B2G nightly both required). It is basic. But already useful, especially to debug Firefox OS apps. For example today, we were looking at the System App and realized that the main thread was never idle. Looking at the timeline, we realized that a restyle was triggered many many times per seconds, even if nothing was happening on the screen. By tweaking the DOM with the inspector, we figured it was coming from a CSS animation that was display:none (see bug 962594).
We are working hard to build new Firefox performance tools. Expect to see better tools coming in the coming months. More info about this new timeline tool here.