The later versions of NoSquint Plus are not compatible with the most recent non-Beta version of Firefox (56.0.1). Instead, they are supposed to be compatible with the beta-version of Firefox 57+, which I have not tested. I rolled back to an earlier version of NoSquint Plus (56.1) and it works fine. Be sure to turn off automatic update for NoSquint Plus, otherwise it will change NoSquint Plus 56.1 with NoSquint Plus 61.3. I would rather not replace my current version of Firefox with the beta-version Firefox 57+.