Developer Edition is now based off Beta so the underlying builds are the same. However there are still differences between the two.
Besides the developer related theme and defaults that Developer Edition has which there are a lot, there are some different build flags. One that jumps out to me is that Developer Edition allows for unsigned add-ons to be run, which means you can still use old Firefox Extensions that won't run anyway else. And there is also a build flag for assertions which helps Firefox developers track down errors in the build by reporting assertions back to a service.