1) The dev cycle (developers, designers, Project Managers, Business Analysts, Release Managers, testers) all have to justify their jobs - what better way than releasing new versions?
2) They have to keep getting "column inches" to stay in the public mind. Every time a new release comes along so does a lot of free column-inches - reviews, hands-ons, blogs on many different websites, online magazines, online blogs, tech sites, forums, software-release sites (app stores, and so on where often 'most recent' is either a sort option or used in automatic ratings/listings).
3) Keeping up with the joneses. The only reason for Firfoxes Australis(I think it was called) UI update was explicitly to make it look more like chrome. Chrome was killing firefox, people were leaving firefox for chrome, so Mozilla decided to look like Chrome in the belief that that was why people were going to Chrome. Of course, once firefox looked like chrome, there was no reason to stay on firefox (for the average user who avoids plugins that can turn FF back into FF UI), as at the time chrome was technically superior from a stability point (separate processes to prevent one site/plugin crashing fromtaking down the whole browser), and the only thing that was keeping a lot of people on FF was the interface. So once that was gone there was no reason to stay on FF for the majority of users (yes plugins were much better on FF, but not that many people in the global scheme of things used the plugins that required FF). And of course, AGAIN Mozilla are breaking this, by making a lot of the plugins that u need FF for, and the reason for staying with FF, break and no longer work, therefore leaving no compelling reason to stay on FF.