Looking at https://www.mozilla.org/en-GB/firefox/developer/ I'm a bit confused... Which features are just new features which will soon be in the main Firefox DevTools, and which are specific to installing the Developer Edition?

AFAIK, Firefox Developer Edition is currently just a repackaging of Firefox Beta and doesn’t have different devtools. It used to be a rebranding of Firefox Aurora (an alpha release channel between Firefox Nightly and Beta, now retired) with a different theme and a few different devtool setting changes.

Maybe I was just unlucky, but the one time I tried it, it refused to launch.

I'd rather not have my professional work tools be built against a beta channel when there are better alternatives.

I have been using the Developer Edition since 2014, when it was released, without issue. You are not locked into a single browser. Personally I use more then one browser, and professionally I always validate against multiple browsers.

I find different browsers excel at different things in regards to the developer experience. As an example I appreciate that Firefox had a formatted JSON view without requiring an extension. However Chrome also has capabilities that I wish were included in Firefox.

What are the differences compared to regular Firefox?

Firefox Developer Edition is targeted towards developers, and contains the latest development tools available.

New features also land quicker, as it is built against Firefox beta.