Starting with Firefox 4, Mozilla has had a new development model that is designed to get features out as soon as possible, in order to make their browser as competitive as possible. Perhaps it is because Mozilla is rapidly losing market share, most notably to Google Chrome: http://www.tested.com/news/firefox-loses-market-share-but-mozilla-doesnt-mind-much/2969/
We should first examine what exactly that new development model is. As of right now, most development of Firefox is done via Mozilla-Central. Development builds of this can be downloaded via Firefox Nightly. Builds of Nightly are highly buggy, but contain the newest features. If the dev team has a problem with a particular part of the next version of Firefox, they simply freeze that component and push the rest through Aurora, Beta, and Release candidates successively. This helps to ensure that a working component of Firefox is never held back from release. Also, a complete explanation can be found here: http://mozilla.github.com/process-releases/draft/development_specifics/
Getting more features faster sounds like a good thing right? Well there are some problems that have become apparent after Mozilla’s switchover. Having 4 builds of Firefox also means that add-ons built for Firefox 4 are not guaranteed to work in Firefox 8, or Firefox 9. Every time Mozilla updates their browser, add-ons may need updating as well. Then there are the security risks that come with using beta, alpha, or even pre-alpha software not designed for release. With a shorter development time, bugs may not become apparent until long after release, allowing for exploits to take advantage of the browser.
For Mozilla at least, the most important question is: “Will this make our browser better for end users?”. As of now, I am not sure.