(see the Finding your profile without opening Firefox section)
Once you have your profile folder located, there should be a prefs.js file inside of it. Open it with a text editor of some kind, find the line containing browser.uiCustomization.state, and delete the whole line.
After that, you should be able to start Firefox Developer Edition again.
Hopefully this is just going to be a temporary workaround and we can rollout a fix very soon.
This arcane workaround has indeed been fixing the problem on my M1 mac already two times. Actually, either a plugin or Firefox itself must have reconfigured as the problem reappeared a few days ago again and gladly I had bookmarked this contribution.
But can it please be fixed? If Mozilla is serious about maintaining or growing the FF market share, I think leaving a problem in their core product for half a year is indeed a show stopper. Surely many others are not searching the entire Internet to edit some .js files on their fs randomly.
I’ve been trying to make it fail on purpose after seeing your post. I set browser.uiCustomization.state to various values (eg strings which would fail parsing, or simply the one shared by mkohler earlier in this thread). But I did not manage to reproduce it yet. Firefox (dev edition) always managed to start correctly (autofixing the preference when I really used non-sensical values).
Let’s see if it happens again for anyone else and if we can get an idea of which value puts the profile in a bad state.