Fixes make it into Aurora fairly regularly, and more rarely new features make it into Aurora after missing the version bump. Release is meant to be more stable than Beta, Beta is meant to be more stable than Aurora, and Aurora is meant to be more stable than Nightly, but the tradeoff is that you have to wait longer for changes (such as new features, performance enhancements and so on).ДЇixes also tend to make it to Nightlies faster, and backporting them to the other branches is only done if the risk/reward ratio is worth it (as a fix for one thing can break another). This way they make sure that people can test all the latest changes, and hopefully enough people will have tested each version before it makes it to release. Nightly turns into Aurora turns into Beta turns into Release when they bump the version number.

And finally there's the latest development version of Firefox, called Nightly, which is currently at version 21 and receives daily updates regardless of what changes may have landed.

Then there's the latest alpha version of Firefox, called Aurora, which is currently at version 20 and updated whenever important fixes happen. Then there's the latest Beta version of Firefox, which is currently at version 19 and is updated periodically (not sure what the period is based on - once every week?). There's the latest released version of Firefox, which is currently at version 18.

Firefox nightly vs developer