Back in the day (and by that I mean a week ago), new versions of Firefox were released “when they were ready” – dat, when all the features they wanted to put in were in it. That’s called feature-based releases, and most developers use that strategy.
But there’s another release strategy that focuses on releasing new versions every so often. Some features might not make a version, but that’s OK; a new version is coming in x weeks so it’ll be included then. This is called fixed releases.