Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
See bug 1372041. I have patches I probably need to rebase now, but should be easy.
We already have the guards in all hot paths we care about, and it will remove a few RwLocks, and the dependency on parking_lot for stylo.