User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.95 Safari/537.36 Steps to reproduce: Attempt to set chrome.runtime.onInstalled listener from background script. onInstalled is undefined Actual results: onInstalled is undefined Expected results: onInstalled should have been defined as per the documentation.
What Firefox version are you using? As noted here: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/runtime/onInstalled#Browser_compatibility the onInstalled event is supported beginning with Firefox 52
Welp, that explains it. Apologies.