This is very bad news for Firefox users like myself who enjoy the browser for its vast library of add-ons, which tend to provide far more functionality than Chrome’s simplistic extensions. Some of the staples that will be broken include NoScript, Tab Mix Plus, and my favorite download manager, DownThemAll!. I think this is a terrible mistake that doesn’t benefit Firefox developers one bit, and people seem to agree.
Firefox add-on developers need to rewrite their add-ons, while Chrome developers can easily port their extensions to Firefox browser with small changes to their code. Who’s benefiting here Chrome developers. This is a huge change, their market share can further decline and Firefox may lose all its loyal users using because of popular add-ons which may not work with the new WebExtensions API.