In the version 1.1.7.8 editor is completely broken. After reverting to previous version it's fixed. Maybe the new theme is wrongly applied or something.
We can't fix what we can't reproduce. Please post a screenshot and an instruction how to make it happen. And if you use FF 56 or older, please re-check it in a new browser profile with no other addons installed because in the old Firefox the full (classic) addons can affect WebExtensions.
