All web browsers have bugs, but when simply viewing a web page can infect your computer with malicious software, the speed with which bugs are found and fixed is critical. It may be the most important yardstick by which to measure any web browser.
Mozilla released version 3.02 of Firefox on Tuesday. It had a bug. Happens all the time. What doesn't happen all the time is that the bug was fixed quickly and version 3.03 of Firefox was released on Friday.
Anyone interested in Defensive Computing doesn't want their bug fixes idling at the gate waiting for the one day a month when they are set free.