Without e10s, the UI is in fact not rendered on its own thread. There was a lot of discussion about whether to try to move the UI to a separate thread or separ

Without e10s, the UI is in fact not rendered on its own thread. There was a lot of discussion about whether to try to move the UI to a separate thread or separate process, and the decision was that separate process has a _much_ lower risk of bugs due to racing memory accesses and whatnot. Basically, it removes a bunch of ways that you could accidentally shoot yourself in the foot while trying to retrofit threading onto a large existing C++ codebase....