Developer edition is essentially Firefox beta, with some tweaks that developers may prefer by default, and some experimental dev features.
As I understand it, it aligns with beta, but is an early beta, meaning that while it shares its codebase, it may get access to some feature rollouts (that are gated by edition) slightly earlier than true Firefox Beta.
Why do you expect that Edge wouldn't adopt Google-like MV3 along with Chrome?
Microsoft adopted Chromium in order to minimise development costs in a product it doesn't see as core, something which would be incurred if it had to maintain its own fork of mv3, and is incentivized through Bing to pursue a similar approach.