On current nightly 56.0a1 (2017-06-20), certain webextensions (such as https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher-revived/?src=search) do not have enough space to show their doorhanger when selected from the chevron overflow menu, cutting off significant functionality. This addon also exhibits the problem of distorted icons, but I believe that that is a dupe of 1367370. I've attached a screenshot of the addon in question.