While adding support for CLJS custom formatters, we constantly bumped into issues with the requirement we set that custom formatters hook need to be side-effect free; CLJS implementation could avoid having side effect, but even with that, we're seeing other issues like Bug 1809613.
As this feature is behind a pref (even 2 at the moment), and given that Chrome doesn't have such requirement, let's use a regular debugger to call the hooks.
The whole feature will go through security review before shipping so we should be safe.
We might have some extra UI to make it more safe (displaying some notification when custom formatters are being used, having some kind of per-domain premission, …)

We can always revisit this in the future if we find that we can handle this with side-effect free debugger

Depends on D167905

https://hg.mozilla.org/mozilla-central/rev/138713ec3f6d