"No well in hell" is a very good answer. Just the HTML DOM and layouting code in KHTML before it was forked into WebKit was an enormously complex piece of code. Then there is the JS implementation (including compiler - JS gets transferred as source code), the SVG DOM and renderer and other goodies that were integral parts of browsers already back then that I'm probably not thinking about.
Just adding to this: the attack surface of a web browser is humongous compared to that of a relatively small runtime, even in 2008. In 2011 when I had a look through the FF codebase, there were already 2 JITs for JavaScript, one which had been in for years (but I don't remember if it was in use).
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