It is perfectly within the bounds of what JavaScript can (and indeed should be able to) do these days, to support e.g. single page applications and smart navigation within them. Therefore, I don't fault Firefox for this, at all.
Web devs are strongly in favor of changes to web browsers that give web devs greater control over the sites that they work on even when those changes (combined with the fact that any psychopath and any irresponsible person can create a web site) worsen the experience of the average user of the web.
Hijacking of the back button is a crisp example of this trend or phenomenon, which is why I jumped into this thread.