(In reply to Philipp Kewisch [:Fallen] [:📆] from comment #5)
Now, putting my Thunderbird hat on: Thunderbird doesn't have OOP enabled, and hence also doesn't use OOP WebExtensions. If falling back to the browser toolbox isn't a major effort to write and maintain, the team would appreciate going that route.
Thanks for the feedback and good point about Thunderbird. I think the TB use case is more critical than Firefox Desktop users who might have turned off OOP extensions. TB extension developers don't have the option to enable OOP extensions. If we disable the Inspect button when OOP extensions are disabled, they won't be able to work on TB addons at all.
To be clear, if we end up reintroducing the browser toolbox flow for addon development, it will most likely be untested and might regress from time to time. The other option would be to start Thunderbird as a remote debugging server, and debug the addons from a Firefox instance.