Sites can prohibit the injection of images and javascript code from extensions (by HTTP header Content-Security-Policy). Mozilla's site including. By this reason part of InFormEnter code (so-called content scripts) are blocked (on problem site press Shift+Ctrl+J and find errors like "Content Security Policy: The page’s settings blocked the loading of a resource at self"). I do not know what it is - a feature or a bug of Firefox (InFormEnter in Chrome allows insert data from context menu on this page). In this situation I can not fix anything. I regret.

Update: it is not a bug, it is a feature. Read here: https://bugzilla.mozilla.org/show_bug.cgi?id=1310082