Hey @Thomas_Buckley-Houston, welcome to the Add-ons section of Mozilla’s Discourse server!
Sorry, I don’t follow the question. Where are you executing the code snippet you shared? There isn’t a ChromeUtils object in extension JS contexts, so attempting to execute the snippet you shared in an extension will throw ReferenceError: ChromeUtils is not defined.
I’m also not clear on what you mean by the “screenshotter from the Web Extensions API.” There’s isn’t “screnshotter” namespace; do you mean browser.runtime.captureVisibleTab()?
Are you perhaps you’re referring to Firefox’s support for writing experimental WebExtensions APIs? If so, I’m not too familiar with them, but I might be able to dig up some info once I get a better picture of what you’re trying to do.