Tools | Options | Software Update | Check Now
should start the download. (It doesn’t appear that the Mac version is ready yet, but use FireFox | Preferences instead of Tools | Options ). If you don’t already have it installed, start at http://www.mozilla.org to check out all the cool products: FireFox, Thunderbird, the Mozilla Suite, Camino and more.
Note that you will need to restart FireFox to complete the re-install. This patch addresses a GIF buffer-overflow security problem, not yet exploited in the wild, supposedly.