The new Mozilla Firefox 19 on Linux has 187 fails on test262.ecmascript.org. That is 22 more fails than Firefox 18, and 20 more fails than Firefox 17 esr. Meanwhile Google Chrome 24 has just 8 fails, Opera 12.14 has only 4, and browsers running WebKitGTK+ 1.10, such as Midori and Web have only 1 fail when I tested it. If, you're running WebkItGTK+ 1.8.3 which is available on Ubuntu 12.04 and Slackware 14.0 via Slackbuilds the number of fails is 112. Although, QtWebKit 2.2 which is part of Qt 4.8 available in most distros has 940 fails, the new QtWebKit 2.3 available in Arch Linux and Chakra has between 3 and 10 fails, depending on which variation is tested. So, once the new QtWebKit becomes standard, Firefox will only be ahead of Konqueror's lightly maintained KHTML in Javascript performance, unless Mozilla gets more focused on improving Javascript accuracy.

seeing as html5 will replace it .....

As far as HTML5 is concerned, WebKitGTK+ 1.10 which is available in Ubuntu 12.10 is almost tied with Firefox, and so is QtWebKit 2.3 beta 2. However, Slackware 14.0, still has WebKitGTK+ 1.8.3 in Slackbuilds which on Slackware has a HTML5 score of 329 compared with the 378 in Firefox 19. Konqueror running QtWebKit 2.2 (Qt 4.8) has an HTML5 score of 319. Google Chrome 24 leads the HTML5 parade with a score of 448. Currently, I have Noop Linux, a lightly used independent distro, on my desktop; and Slackware 14.0 on a laptop. Although, Firefox isn't perfect with scripts, it works fine with almost all the real world sites that I use. Since, I am not sold on Google as a company and the other webkit browsers aren't available or don't work well on Noop Linux, I have decided to browse mostly with Firefox on dynamic sites, although I sometimes use Konqueror on sites that work well with it.