The debugger is empty, no source is displayed when devtools.debugger.features.windowless-service-workers is turned on.
And the following exception happens:

JavaScript error: resource://devtools/server/startup/worker.js, line 37: NetworkError: WorkerDebuggerGlobalScope.loadSubScript: Failed to load worker script at resource://devtools/shared/worker/loader.js (nsresult = 0x805e0006)

This exception is similar to bug 1620023. So we probably have similar failure for some other workers.

My STR is the following:

• Opening the following websites homepages in their own tab (in order to force creating many content processes):
  ** youtube
  ** facebook
  ** wikipedia
  ** leboncoin.fr
  ** twitter
• Toggle the windowless pref to true and open the debugger

AR: the debugger is empty
ER: twitter.com sources are displayed

I was also able to reproduce. This will be a blocker for enabling bug 1603190.

Sounds very similar to https://bugzilla.mozilla.org/show_bug.cgi?id=1618546.
I have a potential patch uploaded there.

Edit: Even if the root cause is the same as Bug 1618546, I think we should still make the debugger more reliable in case service worker debugging throws.

I confirm that the patch from Bug 1618546 fixes this bug as well.

Duping this as fixed by bug 1618546.