Loading extension 'null': Reading manifest: Error processing background.persistent...
in 8.0.9, new warning logged in the console:
addons.webextension. WARN Loading extension 'null': Reading manifest: Error processing background.persistent: Event pages are not currently supported. This will run as a persistent background page.
also:
addons.webextension.{73a6fe31-595d-460b-a920-fcc0f8843232} WARN Loading extension '{73a6fe31-595d-460b-a920-fcc0f8843232}': Reading manifest: Error processing background.persistent: Event pages are not currently supported. This will run as a persistent background page.