We reported during the week on a critical vulnerability in the way Firefox’s 3.5 JIT (just in time) handled JavaScript. Now the Mozilla Foundation has released a new update, bringing Firefox to 3.5.1, fixing the vulnerability.

Mozilla’s definition of a “critical” vulnerability does make the whole thing sound a bit scary, “Vulnerability can be used to run attacker code and install software, requiring no user interaction beyond normal browsing.“ Still, it’s always nice to get a prompt fix, especially when you consider the position this vulnerability had put Firefox users in. Given that the biggest advantage of Firefox 3.5 was its speedier browsing (250% quicker than Firefox 3.0) and since the remedy involved turning off JIT, Firefox users can be forgiven for being a little miffed about the security hole.

Fortunately, with the release of 3.5.1, we can all return to our usual safe-as-possible browsing without taking a hit on speed for security. Moreover, the patch also fixes one particularly frustrating bug with accessing Internet Explorer’s temporary internet files. This had caused the browser to take its sweet time to launch on some Windows machines, but the fix makes the whole experience nice and fast for users for whom it had before been a dithering process before.

We’d advise everyone to do a Firefox update as soon as you get a chance. You can do that by just going to the Help menu and hitting Check for Updates. Otherwise you can always just download the installer here.