I'm not very knowledgeable about Firefox internals, but I recall hearing something like XUL having a C++ backend, and being able to use essentially native code

I'm not very knowledgeable about Firefox internals, but I recall hearing something like XUL having a C++ backend, and being able to use essentially native code to drive UI, extensions (via XPCOM), etc. I figure this is probably faster than rendering a web UI until the SpiderMonkey JIT is sufficiently warm.