privacy.firstparty.isolate=true breaks GM.xmlHttpRequest

[adamhotep]

I get cookies is undefined and Unchecked lastError value: Error: First-Party Isolation is enabled, but the required 'firstPartyDomain' attribute was not set when running a GM userscript calling GM.xmlHttpRequest() with privacy.firstparty.isolate (First-Party Isolation) enabled (set to true):

For now, this means I'm disabling First-Party Isolation, but I'm hoping there's a solution or workaround that will make this work better.

In the event it matters (it shouldn't, I'm on FF 61), I have followed the ghacks workaround to explicitly allow GM to set cookies.

[eternal-sorrow]

Any updates on this?

[adamhotep]

No updates that I'm aware of, @eternal-sorrow. I still experience this problem when privacy.firstparty.isolate is set to true, regardless of the value of its companion setting privacy.firstparty.isolate.restrict_opener_access (now on Firefox 63.0.1).