User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.84 Safari/537.36 Steps to reproduce: 1. Create a basic Firefox Web Extension 2. Update manifest to inject Fast-Levenshtein Library (https://github.com/hiddentao/fast-levenshtein) to all URLs 3. Check for library availability during Script Load and DOM Content Load Attached a zip containing the above changes. Actual results: 4. Levenshtein object available during Script Load, but undefined during DOM Content Load for the many URLs like, - https://www.facebook.com - http://www.cnn.com/ - https://s-usweb.dotomi.com/renderer/ubr/13.0.2/ubr-wrapper.html Expected results: 4. Levenshtein object should be available in global window object both during/after Script Load and DOM Content Load in all URLs