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element.openOrClosedShadowRoot property always returns null

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Hello, I'm trying to use https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/dom/openOrClosedShadowRoot functionality in an extension but the element.openOrClosedShadowRoot always returns null.

Also is there a place in Firefox Devtools console (similar to the Chrome Devtools dropdown) to switch the context from the user console to the extension console so we can try extension only APIs?

Thanks.

Hello, I'm trying to use https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/dom/openOrClosedShadowRoot functionality in an extension but the element.openOrClosedShadowRoot always returns null. Also is there a place in Firefox Devtools console (similar to the Chrome Devtools dropdown) to switch the context from the user console to the extension console so we can try extension only APIs? Thanks.

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Open about:debugging and go to "This Firefox" and click the Inspect button next to the extension.