The source of trouble here is the Web Extension Fallback Document,
which is loaded via the system principal and used by DevTools
as the top level target.
Because of it being system principal, we use the special "devtools loader" for ESM.
This is what cause the exception about the "global" option.
Using global attribute set to contextual fixes this exception.
Because of it being the top level target, we are trying to send the request
from its document. But as it is a system principal, it is ignored by the Network Observer.
So that the resent request is sent, but isn't shown.
Using the currently selected target in Network Command helps circumvent that
by sending the request with a real extension document and the request
is then shown in the network monitor.
But this change also impact web page and browser debugging.
Differential Revision: https://phabricator.services.mozilla.com/D231602
f6cdc992fc