Attaching screenshot.

I never paid any attention to "Add a Keyword for this Search..." but now that I click random form fields, it seems to appear for fields that have nothing to do with searching. They are

<input type="text">

between <form action=URI> and </form> tags.

I haven't yet found a document explaining what Firefox is looking for before it shows that item, but if you don't have an actual form and just have some onclick behavior, that could be the issue.