Chrome adds this property when extensions are uploaded to the chrome store. Since will look at AMO to look for updates, we should ignore this property.

Its a warning, I believe? 1467165402023 addons.webextension.tabs-tabs-tabs-test-1@mozilla.org WARN Loading extension 'tabs-tabs-tabs-test-1@mozilla.org': Reading manifest: Error processing update_url: An unexpected property was found in the WebExtension manifest What more do you propose doing with it?

You're right, I missed that this one was a warning. I think it's fine as is.